xnu-6153.11.26.tar.gz

[apple/xnu.git] / osfmk / kern / turnstile.c

/*
 * Copyright (c) 2017 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

#include <kern/turnstile.h>
#include <kern/cpu_data.h>
#include <kern/mach_param.h>
#include <kern/kern_types.h>
#include <kern/assert.h>
#include <kern/kalloc.h>
#include <kern/thread.h>
#include <kern/clock.h>
#include <kern/policy_internal.h>
#include <kern/task.h>
#include <kern/waitq.h>
#include <kern/sched_prim.h>
#include <kern/zalloc.h>
#include <kern/debug.h>
#include <machine/limits.h>
#include <machine/atomic.h>

#include <pexpert/pexpert.h>
#include <os/hash.h>
#include <libkern/section_keywords.h>

static zone_t turnstiles_zone;
static int turnstile_max_hop;
static struct mpsc_daemon_queue turnstile_deallocate_queue;
#define MAX_TURNSTILES (thread_max)
#define TURNSTILES_CHUNK (THREAD_CHUNK)

/* Global table for turnstile promote policy for all type of turnstiles */
static const turnstile_promote_policy_t turnstile_promote_policy[TURNSTILE_TOTAL_TYPES] = {
        [TURNSTILE_NONE]          = TURNSTILE_PROMOTE_NONE,
        [TURNSTILE_KERNEL_MUTEX]  = TURNSTILE_KERNEL_PROMOTE,
        [TURNSTILE_ULOCK]         = TURNSTILE_USER_PROMOTE,
        [TURNSTILE_PTHREAD_MUTEX] = TURNSTILE_USER_PROMOTE,
        [TURNSTILE_SYNC_IPC]      = TURNSTILE_USER_IPC_PROMOTE,
        [TURNSTILE_WORKLOOPS]     = TURNSTILE_USER_IPC_PROMOTE,
        [TURNSTILE_WORKQS]        = TURNSTILE_USER_IPC_PROMOTE,
        [TURNSTILE_KNOTE]         = TURNSTILE_USER_IPC_PROMOTE,
        [TURNSTILE_SLEEP_INHERITOR] = TURNSTILE_KERNEL_PROMOTE,
};

/* Global table for turnstile hash lock policy for all type of turnstiles */
static const turnstile_hash_lock_policy_t turnstile_hash_lock_policy[TURNSTILE_TOTAL_TYPES] = {
        [TURNSTILE_NONE]          = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_KERNEL_MUTEX]  = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_ULOCK]         = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_PTHREAD_MUTEX] = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_SYNC_IPC]      = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_WORKLOOPS]     = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_WORKQS]        = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_KNOTE]         = TURNSTILE_HASH_LOCK_POLICY_NONE,
        [TURNSTILE_SLEEP_INHERITOR] = (TURNSTILE_IRQ_UNSAFE_HASH | TURNSTILE_LOCKED_HASH),
};

os_refgrp_decl(static, turnstile_refgrp, "turnstile", NULL);

#if DEVELOPMENT || DEBUG
static queue_head_t turnstiles_list;
static lck_spin_t global_turnstile_lock;

lck_grp_t               turnstiles_dev_lock_grp;
lck_attr_t              turnstiles_dev_lock_attr;
lck_grp_attr_t          turnstiles_dev_lock_grp_attr;

#define global_turnstiles_lock_init() \
        lck_spin_init(&global_turnstile_lock, &turnstiles_dev_lock_grp, &turnstiles_dev_lock_attr)
#define global_turnstiles_lock_destroy() \
        lck_spin_destroy(&global_turnstile_lock, &turnstiles_dev_lock_grp)
#define global_turnstiles_lock() \
        lck_spin_lock_grp(&global_turnstile_lock, &turnstiles_dev_lock_grp)
#define global_turnstiles_lock_try() \
        lck_spin_try_lock_grp(&global_turnstile_lock, &turnstiles_dev_lock_grp)
#define global_turnstiles_unlock() \
        lck_spin_unlock(&global_turnstile_lock)

/* Array to store stats for multi-hop boosting */
static struct turnstile_stats turnstile_boost_stats[TURNSTILE_MAX_HOP_DEFAULT] = {};
static struct turnstile_stats turnstile_unboost_stats[TURNSTILE_MAX_HOP_DEFAULT] = {};
uint64_t thread_block_on_turnstile_count;
uint64_t thread_block_on_regular_waitq_count;
#endif /* DEVELOPMENT || DEBUG */

#ifndef max
#define max(a, b)        (((a) > (b)) ? (a) : (b))
#endif /* max */

/* Static function declarations */
static turnstile_type_t
turnstile_get_type(struct turnstile *turnstile);
static uint32_t
turnstile_get_gencount(struct turnstile *turnstile);
static void
turnstile_set_type_and_increment_gencount(struct turnstile *turnstile, turnstile_type_t type);
static void
turnstile_init(struct turnstile *turnstile);
static void
turnstile_update_inheritor_workq_priority_chain(struct turnstile *in_turnstile, spl_t s);
static void
turnstile_update_inheritor_thread_priority_chain(struct turnstile **in_turnstile,
    thread_t *out_thread, int total_hop, turnstile_stats_update_flags_t tsu_flags);
static void
turnstile_update_inheritor_turnstile_priority_chain(struct turnstile **in_out_turnstile,
    int total_hop, turnstile_stats_update_flags_t tsu_flags);
static void
thread_update_waiting_turnstile_priority_chain(thread_t *in_thread,
    struct turnstile **out_turnstile, int thread_hop, int total_hop,
    turnstile_stats_update_flags_t tsu_flags);
static boolean_t
turnstile_update_turnstile_promotion_locked(struct turnstile *dst_turnstile,
    struct turnstile *src_turnstile);
static boolean_t
turnstile_update_turnstile_promotion(struct turnstile *dst_turnstile,
    struct turnstile *src_turnstile);
static boolean_t
turnstile_need_turnstile_promotion_update(struct turnstile *dst_turnstile,
    struct turnstile *src_turnstile);
static boolean_t
turnstile_add_turnstile_promotion(struct turnstile *dst_turnstile,
    struct turnstile *src_turnstile);
static boolean_t
turnstile_remove_turnstile_promotion(struct turnstile *dst_turnstile,
    struct turnstile *src_turnstile);
static boolean_t
turnstile_update_thread_promotion_locked(struct turnstile *dst_turnstile,
    thread_t thread);
static boolean_t
turnstile_need_thread_promotion_update(struct turnstile *dst_turnstile,
    thread_t thread);
static boolean_t
thread_add_turnstile_promotion(
        thread_t thread, struct turnstile *turnstile);
static boolean_t
thread_remove_turnstile_promotion(
        thread_t thread, struct turnstile *turnstile);
static boolean_t
thread_needs_turnstile_promotion_update(thread_t thread,
    struct turnstile *turnstile);
static boolean_t
thread_update_turnstile_promotion(
        thread_t thread, struct turnstile *turnstile);
static boolean_t
thread_update_turnstile_promotion_locked(
        thread_t thread, struct turnstile *turnstile);
static boolean_t
workq_add_turnstile_promotion(
        struct workqueue *wq_inheritor, struct turnstile *turnstile);
static turnstile_stats_update_flags_t
thread_get_update_flags_for_turnstile_propagation_stoppage(thread_t thread);
static turnstile_stats_update_flags_t
turnstile_get_update_flags_for_above_UI_pri_change(struct turnstile *turnstile);
static void turnstile_stash_inheritor(turnstile_inheritor_t new_inheritor,
    turnstile_update_flags_t flags);
static int turnstile_compute_thread_push(struct turnstile *turnstile, thread_t thread);

#if DEVELOPMENT || DEBUG
/* Test primitives and interfaces for testing turnstiles */
struct tstile_test_prim {
        struct turnstile *ttprim_turnstile;
        thread_t ttprim_owner;
        lck_spin_t ttprim_interlock;
        uint32_t tt_prim_waiters;
};

struct tstile_test_prim *test_prim_ts_inline;
struct tstile_test_prim *test_prim_global_htable;
struct tstile_test_prim *test_prim_global_ts_kernel;
struct tstile_test_prim *test_prim_global_ts_kernel_hash;

static void
tstile_test_prim_init(struct tstile_test_prim **test_prim_ptr);
#endif

union turnstile_type_gencount {
        uint32_t value;
        struct {
                uint32_t ts_type:(8 * sizeof(turnstile_type_t)),
                ts_gencount: (8 * (sizeof(uint32_t) - sizeof(turnstile_type_t)));
        };
};

static turnstile_type_t
turnstile_get_type(struct turnstile *turnstile)
{
        union turnstile_type_gencount type_and_gencount;

        type_and_gencount.value = atomic_load_explicit(&turnstile->ts_type_gencount, memory_order_relaxed);
        return (turnstile_type_t) type_and_gencount.ts_type;
}

static uint32_t
turnstile_get_gencount(struct turnstile *turnstile)
{
        union turnstile_type_gencount type_and_gencount;

        type_and_gencount.value = atomic_load_explicit(&turnstile->ts_type_gencount, memory_order_relaxed);
        return (uint32_t) type_and_gencount.ts_gencount;
}

static void
turnstile_set_type_and_increment_gencount(struct turnstile *turnstile, turnstile_type_t type)
{
        union turnstile_type_gencount type_and_gencount;

        /* No need to compare exchange since the store happens under interlock of the primitive */
        type_and_gencount.value = atomic_load_explicit(&turnstile->ts_type_gencount, memory_order_relaxed);
        type_and_gencount.ts_type = type;
        type_and_gencount.ts_gencount++;
        atomic_store_explicit(&turnstile->ts_type_gencount, type_and_gencount.value, memory_order_relaxed);
}


/* Turnstile hashtable Implementation */

/*
 * Maximum number of buckets in the turnstile hashtable. This number affects the
 * performance of the hashtable since it determines the hash collision
 * rate. To experiment with the number of buckets in this hashtable use the
 * "ts_htable_buckets" boot-arg.
 */
#define TURNSTILE_HTABLE_BUCKETS_DEFAULT   32
#define TURNSTILE_HTABLE_BUCKETS_MAX       1024

SLIST_HEAD(turnstile_hashlist, turnstile);

struct turnstile_htable_bucket {
        lck_spin_t                    ts_ht_bucket_lock;
        struct turnstile_hashlist     ts_ht_bucket_list;
};

SECURITY_READ_ONLY_LATE(static uint32_t) ts_htable_buckets;
/* Global hashtable for turnstiles managed with interrupts disabled */
SECURITY_READ_ONLY_LATE(static struct turnstile_htable_bucket *)turnstile_htable_irq_safe;
/* Global hashtable for turnstiles managed with interrupts enabled */
SECURITY_READ_ONLY_LATE(static struct turnstile_htable_bucket *)turnstile_htable;


/* Bucket locks for turnstile hashtable */
lck_grp_t               turnstiles_htable_lock_grp;
lck_attr_t              turnstiles_htable_lock_attr;
lck_grp_attr_t          turnstiles_htable_lock_grp_attr;

#define turnstile_bucket_lock_init(bucket) \
        lck_spin_init(&bucket->ts_ht_bucket_lock, &turnstiles_htable_lock_grp, &turnstiles_htable_lock_attr)
#define turnstile_bucket_lock(bucket) \
        lck_spin_lock_grp(&bucket->ts_ht_bucket_lock, &turnstiles_htable_lock_grp)
#define turnstile_bucket_unlock(bucket) \
        lck_spin_unlock(&bucket->ts_ht_bucket_lock)

#define kdp_turnstile_bucket_is_locked(bucket) \
        kdp_lck_spin_is_acquired(&bucket->ts_ht_bucket_lock)

/*
 * Name: turnstiles_hashtable_init
 *
 * Description: Initializes the global turnstile hash table.
 *
 * Args:
 *   None
 *
 * Returns:
 *   None
 */
static void
turnstiles_hashtable_init(void)
{
        /* Initialize number of buckets in the hashtable */
        if (PE_parse_boot_argn("ts_htable_buckets", &ts_htable_buckets, sizeof(ts_htable_buckets)) != TRUE) {
                ts_htable_buckets = TURNSTILE_HTABLE_BUCKETS_DEFAULT;
        }

        assert(ts_htable_buckets <= TURNSTILE_HTABLE_BUCKETS_MAX);
        uint32_t ts_htable_size = ts_htable_buckets * sizeof(struct turnstile_htable_bucket);
        turnstile_htable_irq_safe = (struct turnstile_htable_bucket *)kalloc(ts_htable_size);
        if (turnstile_htable_irq_safe == NULL) {
                panic("Turnstiles hash table memory allocation failed!");
        }

        turnstile_htable = (struct turnstile_htable_bucket *)kalloc(ts_htable_size);
        if (turnstile_htable == NULL) {
                panic("Turnstiles hash table memory allocation failed!");
        }
        lck_grp_attr_setdefault(&turnstiles_htable_lock_grp_attr);
        lck_grp_init(&turnstiles_htable_lock_grp, "turnstiles_htable_locks", &turnstiles_htable_lock_grp_attr);
        lck_attr_setdefault(&turnstiles_htable_lock_attr);

        /* Initialize all the buckets of the hashtables */
        for (uint32_t i = 0; i < ts_htable_buckets; i++) {
                struct turnstile_htable_bucket *ts_bucket = &(turnstile_htable_irq_safe[i]);
                turnstile_bucket_lock_init(ts_bucket);
                SLIST_INIT(&ts_bucket->ts_ht_bucket_list);

                ts_bucket = &(turnstile_htable[i]);
                turnstile_bucket_lock_init(ts_bucket);
                SLIST_INIT(&ts_bucket->ts_ht_bucket_list);
        }
}

/*
 * Name: turnstile_freelist_empty
 *
 * Description: Checks if the turnstile's freelist is empty
 *              Should be called with the primitive IL held.
 *
 * Args:
 *   Arg1: turnstile
 *
 * Returns:
 *   true if freelist is empty; false otherwise
 */
static inline boolean_t
turnstile_freelist_empty(
        struct turnstile *ts)
{
        return SLIST_EMPTY(&ts->ts_free_turnstiles);
}


/*
 * Name: turnstile_freelist_insert
 *
 * Description: Inserts the turnstile into the freelist of another turnstile
 *              Should be called with the primitive IL held.
 *
 * Args:
 *   Arg1: primitive turnstile
 *   Arg2: turnstile to add to the freelist
 *
 * Returns:
 *   None
 */
static void
turnstile_freelist_insert(
        struct turnstile *dst_ts,
        struct turnstile *free_ts)
{
        assert(turnstile_get_type(dst_ts) == turnstile_get_type(free_ts));
        assert(dst_ts->ts_proprietor == free_ts->ts_proprietor);
        turnstile_state_add(free_ts, TURNSTILE_STATE_FREELIST);
        SLIST_INSERT_HEAD(&dst_ts->ts_free_turnstiles, free_ts, ts_free_elm);
}

/*
 * Name: turnstile_freelist_remove
 *
 * Description: Removes a turnstile from the freelist of a turnstile
 *              Should be called with the primitive IL held.
 *
 * Args:
 *   Arg1: primitive turnstile
 *
 * Returns:
 *   turnstile removed from the freelist
 */
static struct turnstile *
turnstile_freelist_remove(
        struct turnstile *ts)
{
        struct turnstile *ret_turnstile = TURNSTILE_NULL;
        assert(!SLIST_EMPTY(&ts->ts_free_turnstiles));
        ret_turnstile = SLIST_FIRST(&ts->ts_free_turnstiles);
        SLIST_REMOVE_HEAD(&ts->ts_free_turnstiles, ts_free_elm);
        assert(ret_turnstile != TURNSTILE_NULL);
        turnstile_state_remove(ret_turnstile, TURNSTILE_STATE_FREELIST);
        /* Need to initialize the list again, since head and elm are in union */
        SLIST_INIT(&ret_turnstile->ts_free_turnstiles);
        return ret_turnstile;
}

/*
 * Name: turnstile_hash
 *
 * Description: Calculates the hash bucket index for a given proprietor
 *
 * Args:
 *   Arg1: proprietor (key) for hashing
 *
 * Returns:
 *   hash table bucket index for provided proprietor
 */
static inline uint32_t
turnstile_hash(uintptr_t proprietor)
{
        uint32_t hash = os_hash_kernel_pointer((void *)proprietor);
        return hash & (ts_htable_buckets - 1);
}

static inline struct turnstile_htable_bucket *
turnstile_get_bucket(uint32_t index, turnstile_type_t type)
{
        struct turnstile_htable_bucket *ts_bucket;
        int hash_policy = turnstile_hash_lock_policy[type];

        if (hash_policy & TURNSTILE_IRQ_UNSAFE_HASH) {
                ts_bucket = &(turnstile_htable[index]);
        } else {
                ts_bucket = &(turnstile_htable_irq_safe[index]);
        }

        return ts_bucket;
}

/*
 * Name: turnstile_hash_bucket_lock
 *
 * Description: locks the spinlock associated with proprietor's bucket.
 *              if proprietor is specified the index for the hash will be
 *              recomputed and returned in index_proprietor,
 *              otherwise the value save in index_proprietor is used as index.
 *
 * Args:
 *   Arg1: proprietor (key) for hashing
 *   Arg2: index for proprietor in the hash
 *   Arg3: turnstile type
 *
 * Returns: old value of irq if irq were disabled before acquiring the lock.
 */
unsigned
turnstile_hash_bucket_lock(uintptr_t proprietor, uint32_t *index_proprietor, turnstile_type_t type)
{
        struct turnstile_htable_bucket *ts_bucket;
        int hash_policy = turnstile_hash_lock_policy[type];
        bool irq_safe = !(hash_policy & TURNSTILE_IRQ_UNSAFE_HASH);
        spl_t ret = 0;
        uint32_t index;

        /*
         * If the proprietor is specified, the caller doesn't know
         * the index in the hash, so compute it.
         * Otherwise use the value of index provided.
         */
        if (proprietor) {
                index = turnstile_hash(proprietor);
                *index_proprietor = index;
        } else {
                index = *index_proprietor;
        }

        ts_bucket = turnstile_get_bucket(index, type);

        if (irq_safe) {
                ret = splsched();
        }

        turnstile_bucket_lock(ts_bucket);

        return ret;
}

/*
 * Name: turnstile_hash_bucket_unlock
 *
 * Description: unlocks the spinlock associated with proprietor's bucket.
 *              if proprietor is specified the index for the hash will be
 *              recomputed and returned in index_proprietor,
 *              otherwise the value save in index_proprietor is used as index.
 *
 * Args:
 *   Arg1: proprietor (key) for hashing
 *   Arg2: index for proprietor in the hash
 *   Arg3: turnstile type
 *   Arg4: irq value returned by turnstile_hash_bucket_lock
 *
 */
void
turnstile_hash_bucket_unlock(uintptr_t proprietor, uint32_t *index_proprietor, turnstile_type_t type, unsigned s)
{
        struct turnstile_htable_bucket *ts_bucket;
        int hash_policy = turnstile_hash_lock_policy[type];
        bool irq_safe = !(hash_policy & TURNSTILE_IRQ_UNSAFE_HASH);
        uint32_t index;

        /*
         * If the proprietor is specified, the caller doesn't know
         * the index in the hash, so compute it.
         * Otherwise use the value of index provided.
         */
        if (proprietor) {
                index = turnstile_hash(proprietor);
                *index_proprietor = index;
        } else {
                index = *index_proprietor;
        }
        ts_bucket = turnstile_get_bucket(index, type);

        turnstile_bucket_unlock(ts_bucket);
        if (irq_safe) {
                splx(s);
        }
}

/*
 * Name: turnstile_htable_lookup_add
 *
 * Description: Lookup the proprietor in the global turnstile hash table.
 *              If an entry is present, add the new turnstile to the entry's freelist.
 *              Otherwise add the passed in turnstile for that proprietor.
 *              The routine assumes that the turnstile->proprietor does not change
 *              while the turnstile is in the global hash table.
 *
 * Args:
 *   Arg1: proprietor
 *   Arg2: new turnstile for primitive
 *   Arg3: turnstile_type_t type
 *
 * Returns:
 *   Previous turnstile for proprietor in the hash table
 */
static struct turnstile *
turnstile_htable_lookup_add(
        uintptr_t proprietor,
        struct turnstile *new_turnstile,
        turnstile_type_t type)
{
        uint32_t index = turnstile_hash(proprietor);
        assert(index < ts_htable_buckets);
        struct turnstile_htable_bucket *ts_bucket;
        int hash_policy = turnstile_hash_lock_policy[type];
        bool needs_lock = !(hash_policy & TURNSTILE_LOCKED_HASH);
        bool irq_safe = !(hash_policy & TURNSTILE_IRQ_UNSAFE_HASH);
        spl_t s;

        ts_bucket = turnstile_get_bucket(index, type);

        if (needs_lock) {
                if (irq_safe) {
                        s = splsched();
                }
                turnstile_bucket_lock(ts_bucket);
        }

        struct turnstile *ts;

        SLIST_FOREACH(ts, &ts_bucket->ts_ht_bucket_list, ts_htable_link) {
                if (ts->ts_proprietor == proprietor) {
                        /*
                         * Found an entry in the hashtable for this proprietor; add thread turnstile to freelist
                         * and return this turnstile
                         */
                        if (needs_lock) {
                                turnstile_bucket_unlock(ts_bucket);
                                if (irq_safe) {
                                        splx(s);
                                }
                        }
                        turnstile_freelist_insert(ts, new_turnstile);
                        return ts;
                }
        }

        /* No entry for this proprietor; add the new turnstile in the hash table */
        SLIST_INSERT_HEAD(&ts_bucket->ts_ht_bucket_list, new_turnstile, ts_htable_link);
        turnstile_state_add(new_turnstile, TURNSTILE_STATE_HASHTABLE);
        if (needs_lock) {
                turnstile_bucket_unlock(ts_bucket);
                if (irq_safe) {
                        splx(s);
                }
        }
        /* Since there was no previous entry for this proprietor, return TURNSTILE_NULL */
        return TURNSTILE_NULL;
}

/*
 * Name: turnstable_htable_lookup_remove
 *
 * Description: Lookup the proprietor in the global turnstile hash table.
 *              For the turnstile in the hash table, if the freelist has turnstiles on it
 *              return one of them from the freelist. Otherwise remove the turnstile from
 *              the hashtable and return that.
 *              The routine assumes that the turnstile->proprietor does not change
 *              while the turnstile is in the global hash table.
 *
 * Args:
 *   Arg1: proprietor
 *   Arg2: free turnstile to be returned
 *   Arg3: turnstile_type_t type
 *
 * Returns:
 *   turnstile for this proprietor in the hashtable after the removal
 */
static struct turnstile *
turnstable_htable_lookup_remove(
        uintptr_t proprietor,
        struct turnstile **free_turnstile,
        turnstile_type_t type)
{
        uint32_t index = turnstile_hash(proprietor);
        assert(index < ts_htable_buckets);
        struct turnstile_htable_bucket *ts_bucket;
        struct turnstile *ret_turnstile = TURNSTILE_NULL;
        int hash_policy = turnstile_hash_lock_policy[type];
        bool needs_lock = !(hash_policy & TURNSTILE_LOCKED_HASH);
        bool irq_safe = !(hash_policy & TURNSTILE_IRQ_UNSAFE_HASH);
        spl_t s;

        ts_bucket = turnstile_get_bucket(index, type);

        if (needs_lock) {
                if (irq_safe) {
                        s = splsched();
                }
                turnstile_bucket_lock(ts_bucket);
        }

        struct turnstile *ts, **prev_tslink;
        /* Find the turnstile for the given proprietor in the hashtable */
        SLIST_FOREACH_PREVPTR(ts, prev_tslink, &ts_bucket->ts_ht_bucket_list, ts_htable_link) {
                if (ts->ts_proprietor == proprietor) {
                        ret_turnstile = ts;
                        break;
                }
        }
        assert(ret_turnstile != TURNSTILE_NULL);

        /* Check if the turnstile has any turnstiles on its freelist */
        if (turnstile_freelist_empty(ret_turnstile)) {
                /* No turnstiles on the freelist; remove the turnstile from the hashtable and mark it freed */
                *prev_tslink = SLIST_NEXT(ret_turnstile, ts_htable_link);
                turnstile_state_remove(ret_turnstile, TURNSTILE_STATE_HASHTABLE);
                if (needs_lock) {
                        turnstile_bucket_unlock(ts_bucket);
                        if (irq_safe) {
                                splx(s);
                        }
                }
                *free_turnstile = ret_turnstile;
                return TURNSTILE_NULL;
        } else {
                /*
                 * Turnstile has free turnstiles on its list; leave the hashtable unchanged
                 * and return the first turnstile in the freelist as the free turnstile
                 */
                if (needs_lock) {
                        turnstile_bucket_unlock(ts_bucket);
                        if (irq_safe) {
                                splx(s);
                        }
                }
                *free_turnstile = turnstile_freelist_remove(ret_turnstile);
                return ret_turnstile;
        }
}

/*
 * Name: turnstile_htable_lookup
 *
 * Description: Lookup the proprietor in the global turnstile hash table.
 *              The routine assumes that the turnstile->proprietor does not change
 *              while the turnstile is in the global hash table.
 *
 * Args:
 *   Arg1: proprietor
 *   Arg2: turnstile_type_t type
 *
 * Returns:
 *   Turnstile for proprietor in the hash table
 */
static struct turnstile *
turnstile_htable_lookup(
        uintptr_t proprietor,
        turnstile_type_t type)
{
        uint32_t index = turnstile_hash(proprietor);
        assert(index < ts_htable_buckets);
        bool kdp_ctx = !not_in_kdp;
        struct turnstile_htable_bucket *ts_bucket = turnstile_get_bucket(index, type);
        int hash_policy = turnstile_hash_lock_policy[type];
        bool needs_lock = !(hash_policy & TURNSTILE_LOCKED_HASH);
        bool irq_safe = !(hash_policy & TURNSTILE_IRQ_UNSAFE_HASH);
        spl_t s;

        if (needs_lock) {
                if (irq_safe && !kdp_ctx) {
                        s = splsched();
                }

                if (kdp_ctx) {
                        if (kdp_turnstile_bucket_is_locked(ts_bucket)) {
                                /* This should move to TURNSTILE_BUSY once 51725781 is in the build */
                                return TURNSTILE_NULL;
                        }
                } else {
                        turnstile_bucket_lock(ts_bucket);
                }
        }
        struct turnstile *ts = TURNSTILE_NULL;
        struct turnstile *ret_turnstile = TURNSTILE_NULL;

        SLIST_FOREACH(ts, &ts_bucket->ts_ht_bucket_list, ts_htable_link) {
                if (ts->ts_proprietor == proprietor) {
                        /* Found an entry in the hashtable for this proprietor */
                        ret_turnstile = ts;
                        break;
                }
        }

        if (needs_lock && !kdp_ctx) {
                turnstile_bucket_unlock(ts_bucket);
                if (irq_safe) {
                        splx(s);
                }
        }

        return ret_turnstile;
}

/*
 * Name: turnstile_deallocate_queue_invoke
 *
 * Description: invoke function for the asynchronous turnstile deallocation
 *              queue
 *
 * Arg1: &turnstile_deallocate_queue
 * Arg2: a pointer to the turnstile ts_deallocate_link member of a tunrstile to
 *       destroy.
 *
 * Returns: None.
 */
static void
turnstile_deallocate_queue_invoke(mpsc_queue_chain_t e,
    __assert_only mpsc_daemon_queue_t dq)
{
        struct turnstile *ts;

        ts = mpsc_queue_element(e, struct turnstile, ts_deallocate_link);
        assert(dq == &turnstile_deallocate_queue);
        turnstile_destroy(ts);
}

/*
 * Name: turnstiles_init
 *
 * Description: Initialize turnstile sub system.
 *
 * Args: None.
 *
 * Returns: None.
 */
void
turnstiles_init(void)
{
        turnstiles_zone = zinit(sizeof(struct turnstile),
            MAX_TURNSTILES * sizeof(struct turnstile),
            TURNSTILES_CHUNK * sizeof(struct turnstile),
            "turnstiles");

        if (!PE_parse_boot_argn("turnstile_max_hop", &turnstile_max_hop, sizeof(turnstile_max_hop))) {
                turnstile_max_hop = TURNSTILE_MAX_HOP_DEFAULT;
        }

        turnstiles_hashtable_init();

        thread_deallocate_daemon_register_queue(&turnstile_deallocate_queue,
            turnstile_deallocate_queue_invoke);

#if DEVELOPMENT || DEBUG
        /* Initialize the global turnstile locks and lock group */

        lck_grp_attr_setdefault(&turnstiles_dev_lock_grp_attr);
        lck_grp_init(&turnstiles_dev_lock_grp, "turnstiles_dev_lock", &turnstiles_dev_lock_grp_attr);
        lck_attr_setdefault(&turnstiles_dev_lock_attr);
        global_turnstiles_lock_init();

        queue_init(&turnstiles_list);

        /* Initialize turnstile test primitive */
        tstile_test_prim_init(&test_prim_ts_inline);
        tstile_test_prim_init(&test_prim_global_htable);
        tstile_test_prim_init(&test_prim_global_ts_kernel);
        tstile_test_prim_init(&test_prim_global_ts_kernel_hash);
#endif
        return;
}

/*
 * Name: turnstile_alloc
 *
 * Description: Allocate a turnstile.
 *
 * Args: None.
 *
 * Returns:
 *   turnstile on Success.
 */
struct turnstile *
turnstile_alloc(void)
{
        struct turnstile *turnstile = TURNSTILE_NULL;

        turnstile = zalloc(turnstiles_zone);
        turnstile_init(turnstile);

#if DEVELOPMENT || DEBUG
        /* Add turnstile to global list */
        global_turnstiles_lock();
        queue_enter(&turnstiles_list, turnstile,
            struct turnstile *, ts_global_elm);
        global_turnstiles_unlock();
#endif
        return turnstile;
}

/*
 * Name: turnstile_init
 *
 * Description: Initialize the turnstile.
 *
 * Args:
 *   Arg1: turnstile to initialize
 *
 * Returns: None.
 */
static void
turnstile_init(struct turnstile *turnstile)
{
        kern_return_t kret;

        /* Initialize the waitq */
        kret = waitq_init(&turnstile->ts_waitq, SYNC_POLICY_DISABLE_IRQ | SYNC_POLICY_REVERSED |
            SYNC_POLICY_TURNSTILE);
        assert(kret == KERN_SUCCESS);

        turnstile->ts_inheritor = TURNSTILE_INHERITOR_NULL;
        SLIST_INIT(&turnstile->ts_free_turnstiles);
        os_atomic_init(&turnstile->ts_type_gencount, 0);
        turnstile_set_type_and_increment_gencount(turnstile, TURNSTILE_NONE);
        turnstile_state_init(turnstile, TURNSTILE_STATE_THREAD);
        os_ref_init_count(&turnstile->ts_refcount, &turnstile_refgrp, 1);
        turnstile->ts_proprietor = TURNSTILE_PROPRIETOR_NULL;
        turnstile->ts_priority = 0;
        turnstile->ts_inheritor_flags = TURNSTILE_UPDATE_FLAGS_NONE;
        turnstile->ts_port_ref = 0;
        priority_queue_init(&turnstile->ts_inheritor_queue,
            PRIORITY_QUEUE_BUILTIN_MAX_HEAP);

#if DEVELOPMENT || DEBUG
        turnstile->ts_thread = current_thread();
        turnstile->ts_prev_thread = NULL;
#endif
}

/*
 * Name: turnstile_reference
 *
 * Description: Take a reference on the turnstile.
 *
 * Arg1: turnstile
 *
 * Returns: None.
 */
void
turnstile_reference(struct turnstile *turnstile)
{
        if (turnstile == TURNSTILE_NULL) {
                return;
        }
        os_ref_retain(&turnstile->ts_refcount);
}

/*
 * Name: turnstile_deallocate
 *
 * Description: Drop a reference on the turnstile.
 *              Destroy the turnstile if the last ref.
 *
 * Arg1: turnstile
 *
 * Returns: None.
 */
void
turnstile_deallocate(struct turnstile *turnstile)
{
        if (turnstile == TURNSTILE_NULL) {
                return;
        }

        if (__improbable(os_ref_release(&turnstile->ts_refcount) == 0)) {
                turnstile_destroy(turnstile);
        }
}

/*
 * Name: turnstile_deallocate_safe
 *
 * Description: Drop a reference on the turnstile safely without triggering zfree.
 *
 * Arg1: turnstile
 *
 * Returns: None.
 */
void
turnstile_deallocate_safe(struct turnstile *turnstile)
{
        if (turnstile == TURNSTILE_NULL) {
                return;
        }

        if (__improbable(os_ref_release(&turnstile->ts_refcount) == 0)) {
                mpsc_daemon_enqueue(&turnstile_deallocate_queue,
                    &turnstile->ts_deallocate_link, MPSC_QUEUE_DISABLE_PREEMPTION);
        }
}

/*
 * Name: turnstile_destroy
 *
 * Description: Deallocates the turnstile.
 *
 * Args:
 *   Arg1: turnstile
 *
 * Returns: None.
 */
void
turnstile_destroy(struct turnstile *turnstile)
{
        /* destroy the waitq */
        waitq_deinit(&turnstile->ts_waitq);

        assert(turnstile->ts_inheritor == TURNSTILE_INHERITOR_NULL);
        assert(SLIST_EMPTY(&turnstile->ts_free_turnstiles));
        assert(turnstile->ts_state & TURNSTILE_STATE_THREAD);
#if DEVELOPMENT || DEBUG
        /* Remove turnstile from global list */
        global_turnstiles_lock();
        queue_remove(&turnstiles_list, turnstile,
            struct turnstile *, ts_global_elm);
        global_turnstiles_unlock();
#endif
        zfree(turnstiles_zone, turnstile);
}

/*
 * Name: turnstile_prepare
 *
 * Description: Transfer current thread's turnstile to primitive or it's free turnstile list.
 *              Function is called holding the interlock (spinlock) of the primitive.
 *              The turnstile returned by this function is safe to use untill the thread calls turnstile_complete.
 *              When no turnstile is provided explicitly, the calling thread will not have a turnstile attached to
 *              it untill it calls turnstile_complete.
 *
 * Args:
 *   Arg1: proprietor
 *   Arg2: pointer in primitive struct to store turnstile
 *   Arg3: turnstile to use instead of taking it from thread.
 *   Arg4: type of primitive
 *
 * Returns:
 *   turnstile.
 */
struct turnstile *
turnstile_prepare(
        uintptr_t proprietor,
        struct turnstile **tstore,
        struct turnstile *turnstile,
        turnstile_type_t type)
{
        thread_t thread = current_thread();
        struct turnstile *ret_turnstile = TURNSTILE_NULL;
        struct turnstile *thread_turnstile = turnstile;

        /* Get the thread's turnstile if no turnstile provided */
        if (thread_turnstile == TURNSTILE_NULL) {
                thread_turnstile = thread->turnstile;
                assert(thread_turnstile != TURNSTILE_NULL);
                assert(thread->inheritor == NULL);
                thread->turnstile = TURNSTILE_NULL;
        }

        /* Prepare the thread turnstile to be the primitive turnstile */
        SLIST_INIT(&thread_turnstile->ts_free_turnstiles);
        turnstile_set_type_and_increment_gencount(thread_turnstile, type);
        thread_turnstile->ts_inheritor = TURNSTILE_INHERITOR_NULL;
        thread_turnstile->ts_proprietor = proprietor;
        turnstile_state_remove(thread_turnstile, TURNSTILE_STATE_THREAD);

        thread_turnstile->ts_priority = 0;
#if DEVELOPMENT || DEBUG
        thread_turnstile->ts_prev_thread = thread_turnstile->ts_thread;
        thread_turnstile->ts_thread = NULL;
#endif

        if (tstore != NULL) {
                /*
                 * If the primitive stores the turnstile,
                 * If there is already a turnstile, put the thread_turnstile if the primitive currently does not have a
                 * turnstile.
                 * Else, add the thread turnstile to freelist of the primitive turnstile.
                 */
                ret_turnstile = *tstore;
                if (*tstore == TURNSTILE_NULL) {
                        turnstile_state_add(thread_turnstile, TURNSTILE_STATE_PROPRIETOR);
                        *tstore = thread_turnstile;
                        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                            (TURNSTILE_CODE(TURNSTILE_FREELIST_OPERATIONS, (TURNSTILE_PREPARE))) | DBG_FUNC_NONE,
                            VM_KERNEL_UNSLIDE_OR_PERM(thread_turnstile),
                            VM_KERNEL_UNSLIDE_OR_PERM(proprietor),
                            turnstile_get_type(thread_turnstile), 0, 0);
                } else {
                        turnstile_freelist_insert(ret_turnstile, thread_turnstile);
                }
                ret_turnstile = *tstore;
        } else {
                /*
                 * Lookup the primitive in the turnstile hash table and see if it already has an entry.
                 */
                ret_turnstile = turnstile_htable_lookup_add(proprietor, thread_turnstile, type);
                if (ret_turnstile == NULL) {
                        ret_turnstile = thread_turnstile;
                        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                            (TURNSTILE_CODE(TURNSTILE_FREELIST_OPERATIONS, (TURNSTILE_PREPARE))) | DBG_FUNC_NONE,
                            VM_KERNEL_UNSLIDE_OR_PERM(thread_turnstile),
                            VM_KERNEL_UNSLIDE_OR_PERM(proprietor),
                            turnstile_get_type(thread_turnstile), 0, 0);
                }
        }

        return ret_turnstile;
}

/*
 * Name: turnstile_complete
 *
 * Description: Transfer the primitive's turnstile or from it's freelist to current thread.
 *              Current thread will have a turnstile attached to it after this call.
 *
 * Args:
 *   Arg1: proprietor
 *   Arg2: pointer in primitive struct to update turnstile
 *   Arg3: pointer to store the returned turnstile instead of attaching it to thread
 *   Arg4: type of primitive
 *
 * Returns:
 *   None.
 */
void
turnstile_complete(
        uintptr_t proprietor,
        struct turnstile **tstore,
        struct turnstile **out_turnstile,
        turnstile_type_t type)
{
        thread_t thread = current_thread();
        struct turnstile *primitive_turnstile = TURNSTILE_NULL;
        struct turnstile *thread_turnstile = TURNSTILE_NULL;

        assert(thread->inheritor == NULL);

        if (tstore != NULL) {
                /*
                 * If the primitive stores the turnstile, check if the primitive turnstile
                 * has any turnstiles on its freelist.
                 */
                assert(*tstore != TURNSTILE_NULL);
                if (turnstile_freelist_empty(*tstore)) {
                        /* Last turnstile scenario; remove the primitive->turnstile */
                        thread_turnstile = *tstore;
                        *tstore = TURNSTILE_NULL;
                        turnstile_state_remove(thread_turnstile, TURNSTILE_STATE_PROPRIETOR);
                } else {
                        /* Freelist has turnstiles; remove one from the freelist */
                        thread_turnstile = turnstile_freelist_remove(*tstore);
                }
                primitive_turnstile = *tstore;
        } else {
                /* Use the global hash to find and remove a turnstile */
                primitive_turnstile = turnstable_htable_lookup_remove(proprietor, &thread_turnstile, type);
        }
        if (primitive_turnstile == NULL) {
                /*
                 * Primitive no longer has a turnstile associated with it, thread_turnstile
                 * was the last turnstile attached to primitive, clear out the inheritor and
                 * set the old inheritor for turnstile cleanup.
                 */
                if (thread_turnstile->ts_inheritor != TURNSTILE_INHERITOR_NULL) {
                        turnstile_update_inheritor(thread_turnstile, TURNSTILE_INHERITOR_NULL,
                            (TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_THREAD));
                        /*
                         * old inheritor is set in curret thread and its priority propagation
                         * will happen in turnstile cleanup call
                         */
                }
                assert(thread_turnstile->ts_inheritor == TURNSTILE_INHERITOR_NULL);

                KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                    (TURNSTILE_CODE(TURNSTILE_FREELIST_OPERATIONS, (TURNSTILE_COMPLETE))) | DBG_FUNC_NONE,
                    VM_KERNEL_UNSLIDE_OR_PERM(thread_turnstile),
                    VM_KERNEL_UNSLIDE_OR_PERM(proprietor),
                    turnstile_get_type(thread_turnstile), 0, 0);
        } else {
                /* If primitive's turnstile needs priority update, set it up for turnstile cleanup */
                if (turnstile_recompute_priority(primitive_turnstile)) {
                        turnstile_reference(primitive_turnstile);
                        thread->inheritor = primitive_turnstile;
                        thread->inheritor_flags = (TURNSTILE_INHERITOR_TURNSTILE |
                            TURNSTILE_INHERITOR_NEEDS_PRI_UPDATE);
                }
        }

        turnstile_set_type_and_increment_gencount(thread_turnstile, TURNSTILE_NONE);
#if DEVELOPMENT || DEBUG
        thread_turnstile->ts_prev_thread = NULL;
        thread_turnstile->ts_thread = thread;
#endif

        turnstile_state_add(thread_turnstile, TURNSTILE_STATE_THREAD);
        if (out_turnstile == NULL) {
                /* Prepare the turnstile to become the thread's turnstile */
                thread->turnstile = thread_turnstile;
        } else {
                *out_turnstile = thread_turnstile;
        }
        return;
}

/*
 * Name: turnstile_kernel_update_inheritor_on_wake_locked
 *
 * Description: Set thread as the inheritor of the turnstile and
 *              boost the inheritor.
 * Args:
 *   Arg1: turnstile
 *   Arg2: new_inheritor
 *   Arg3: flags
 *
 * Called with turnstile locked
 */
void
turnstile_kernel_update_inheritor_on_wake_locked(
        struct turnstile *turnstile,
        turnstile_inheritor_t new_inheritor,
        turnstile_update_flags_t flags __assert_only)
{
        /* for now only kernel primitives are allowed to call this function */
        __assert_only turnstile_promote_policy_t policy =
            turnstile_promote_policy[turnstile_get_type(turnstile)];

        assert(flags & TURNSTILE_INHERITOR_THREAD);
        assert(policy == TURNSTILE_KERNEL_PROMOTE || policy == TURNSTILE_USER_PROMOTE);

        turnstile_stash_inheritor((thread_t)new_inheritor, TURNSTILE_INHERITOR_THREAD);
        /*
         * new_inheritor has just been removed from the turnstile waitq,
         * the turnstile new priority needs to be recomputed so that
         * when new_inheritor will become this turnstile inheritor can
         * inherit the correct priority.
         */
        turnstile_recompute_priority_locked(turnstile);
        turnstile_update_inheritor_locked(turnstile);
}

/*
 * Name: turnstile_update_inheritor_locked
 *
 * Description: Update the inheritor of the turnstile and boost the
 *              inheritor, called with turnstile locked.
 *
 * Args:
 *   Arg1: turnstile
 *   Implicit arg: new inheritor value is stashed in current thread's struct
 *
 * Returns:
 *   old inheritor reference is returned on current thread's struct.
 */
void
turnstile_update_inheritor_locked(
        struct turnstile *turnstile)
{
        turnstile_inheritor_t old_inheritor = turnstile->ts_inheritor;
        turnstile_update_flags_t old_inheritor_flags = turnstile->ts_inheritor_flags;
        thread_t thread = current_thread();
        boolean_t old_inheritor_needs_update = FALSE;
        boolean_t new_inheritor_needs_update = FALSE;
        turnstile_stats_update_flags_t tsu_flags =
            turnstile_get_update_flags_for_above_UI_pri_change(turnstile);

        assert(waitq_held(&turnstile->ts_waitq));

        /*
         * Get the new inheritor value from current thread's
         * struct, the value was stashed by turnstile_update_inheritor
         */
        turnstile_inheritor_t new_inheritor = thread->inheritor;
        turnstile_update_flags_t new_inheritor_flags = thread->inheritor_flags;

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
                break;
        case TURNSTILE_KERNEL_PROMOTE:
                /* some sanity checks, turnstile kernel can push just between threads */
                if (old_inheritor) {
                        assert(old_inheritor_flags & TURNSTILE_INHERITOR_THREAD);
                }

                if (new_inheritor) {
                        assert(new_inheritor_flags & TURNSTILE_INHERITOR_THREAD);
                }

                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        /* Check if update is needed */
        if (old_inheritor == new_inheritor && old_inheritor == NULL) {
                goto done;
        }

        if (old_inheritor == new_inheritor) {
                if (new_inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                        thread_t thread_inheritor = (thread_t)new_inheritor;

                        assert(old_inheritor_flags & TURNSTILE_INHERITOR_THREAD);

                        /* adjust turnstile position in the thread's inheritor list */
                        new_inheritor_needs_update = thread_update_turnstile_promotion(
                                thread_inheritor, turnstile);
                } else if (new_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                        struct turnstile *inheritor_turnstile = new_inheritor;

                        assert(old_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE);

                        new_inheritor_needs_update = turnstile_update_turnstile_promotion(
                                inheritor_turnstile, turnstile);
                } else if (new_inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                        /*
                         * When we are still picking "WORKQ" then possible racing
                         * updates will call redrive through their own propagation
                         * and we don't need to update anything here.
                         */
                        turnstile_stats_update(1, TSU_NO_PRI_CHANGE_NEEDED |
                            TSU_TURNSTILE_ARG | TSU_BOOST_ARG, turnstile);
                } else {
                        panic("Inheritor flags lost along the way");
                }

                /* Update turnstile stats */
                if (!new_inheritor_needs_update) {
                        turnstile_stats_update(1, TSU_PRI_PROPAGATION |
                            TSU_TURNSTILE_ARG | TSU_BOOST_ARG | tsu_flags, turnstile);
                }
                goto done;
        }

        if (old_inheritor != NULL) {
                if (old_inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                        thread_t thread_inheritor = (thread_t)old_inheritor;

                        /* remove turnstile from thread's inheritor list */
                        old_inheritor_needs_update = thread_remove_turnstile_promotion(thread_inheritor, turnstile);
                } else if (old_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                        struct turnstile *old_turnstile = old_inheritor;

                        old_inheritor_needs_update = turnstile_remove_turnstile_promotion(
                                old_turnstile, turnstile);
                } else if (old_inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                        /*
                         * We don't need to do anything when the push was WORKQ
                         * because nothing is pushed on in the first place.
                         */
                        turnstile_stats_update(1, TSU_NO_PRI_CHANGE_NEEDED |
                            TSU_TURNSTILE_ARG, turnstile);
                } else {
                        panic("Inheritor flags lost along the way");
                }
                /* Update turnstile stats */
                if (!old_inheritor_needs_update) {
                        turnstile_stats_update(1, TSU_PRI_PROPAGATION | TSU_TURNSTILE_ARG,
                            turnstile);
                }
        }

        if (new_inheritor != NULL) {
                if (new_inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                        thread_t thread_inheritor = (thread_t)new_inheritor;

                        assert(new_inheritor_flags & TURNSTILE_INHERITOR_THREAD);
                        /* add turnstile to thread's inheritor list */
                        new_inheritor_needs_update = thread_add_turnstile_promotion(
                                thread_inheritor, turnstile);
                } else if (new_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                        struct turnstile *new_turnstile = new_inheritor;

                        new_inheritor_needs_update = turnstile_add_turnstile_promotion(
                                new_turnstile, turnstile);
                } else if (new_inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                        struct workqueue *wq_inheritor = new_inheritor;

                        new_inheritor_needs_update = workq_add_turnstile_promotion(
                                wq_inheritor, turnstile);
                        if (!new_inheritor_needs_update) {
                                turnstile_stats_update(1, TSU_NO_PRI_CHANGE_NEEDED |
                                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG, turnstile);
                        }
                } else {
                        panic("Inheritor flags lost along the way");
                }
                /* Update turnstile stats */
                if (!new_inheritor_needs_update) {
                        turnstile_stats_update(1, TSU_PRI_PROPAGATION |
                            TSU_TURNSTILE_ARG | TSU_BOOST_ARG | tsu_flags, turnstile);
                }
        }

done:
        if (old_inheritor_needs_update) {
                old_inheritor_flags |= TURNSTILE_INHERITOR_NEEDS_PRI_UPDATE;
        }

        /*
         * If new inheritor needs priority updated, then set TURNSTILE_NEEDS_PRI_UPDATE
         * on the old_inheritor_flags which will be copied to the thread.
         */
        if (new_inheritor_needs_update) {
                old_inheritor_flags |= TURNSTILE_NEEDS_PRI_UPDATE;
        }

        turnstile->ts_inheritor = new_inheritor;
        turnstile->ts_inheritor_flags = new_inheritor_flags;
        thread->inheritor = old_inheritor;
        thread->inheritor_flags = old_inheritor_flags;
        return;
}

/*
 * Name: turnstile_stash_inheritor
 *
 * Description: Save the new inheritor reference of the turnstile on the
 *              current thread. It will take a thread reference on the inheritor.
 *              Called with the interlock of the primitive held.
 *
 * Args:
 *   Arg1: inheritor
 *   Arg2: flags
 *
 * Returns:
 *   old inheritor reference is stashed on current thread's struct.
 */
static void
turnstile_stash_inheritor(
        turnstile_inheritor_t new_inheritor,
        turnstile_update_flags_t flags)
{
        thread_t thread = current_thread();

        /*
         * Set the inheritor on calling thread struct, no need
         * to take the turnstile waitq lock since the inheritor
         * is protected by the primitive's interlock
         */
        assert(thread->inheritor == TURNSTILE_INHERITOR_NULL);
        thread->inheritor = new_inheritor;
        thread->inheritor_flags = TURNSTILE_UPDATE_FLAGS_NONE;
        if (new_inheritor == TURNSTILE_INHERITOR_NULL) {
                /* nothing to retain or remember */
        } else if (flags & TURNSTILE_INHERITOR_THREAD) {
                thread->inheritor_flags |= TURNSTILE_INHERITOR_THREAD;
                thread_reference((thread_t)new_inheritor);
        } else if (flags & TURNSTILE_INHERITOR_TURNSTILE) {
                thread->inheritor_flags |= TURNSTILE_INHERITOR_TURNSTILE;
                turnstile_reference((struct turnstile *)new_inheritor);
        } else if (flags & TURNSTILE_INHERITOR_WORKQ) {
                thread->inheritor_flags |= TURNSTILE_INHERITOR_WORKQ;
                workq_reference((struct workqueue *)new_inheritor);
        } else {
                panic("Missing type in flags (%x) for inheritor (%p)", flags,
                    new_inheritor);
        }
}

/*
 * Name: turnstile_update_inheritor
 *
 * Description: Update the inheritor of the turnstile and boost the
 *              inheritor. It will take a thread reference on the inheritor.
 *              Called with the interlock of the primitive held.
 *
 * Args:
 *   Arg1: turnstile
 *   Arg2: inheritor
 *   Arg3: flags - TURNSTILE_DELAYED_UPDATE - update will happen later in assert_wait
 *
 * Returns:
 *   old inheritor reference is stashed on current thread's struct.
 */
void
turnstile_update_inheritor(
        struct turnstile *turnstile,
        turnstile_inheritor_t new_inheritor,
        turnstile_update_flags_t flags)
{
        spl_t spl;

        turnstile_stash_inheritor(new_inheritor, flags);

        /* Do not perform the update if delayed update is specified */
        if (flags & TURNSTILE_DELAYED_UPDATE) {
                return;
        }

        /* lock the turnstile waitq */
        spl = splsched();
        waitq_lock(&turnstile->ts_waitq);

        turnstile_update_inheritor_locked(turnstile);

        waitq_unlock(&turnstile->ts_waitq);
        splx(spl);

        return;
}


/*
 * Name: turnstile_need_thread_promotion_update
 *
 * Description: Check if thread's place in the turnstile waitq needs to be updated.
 *
 * Arg1: dst turnstile
 * Arg2: thread
 *
 * Returns: TRUE: if turnstile_update_thread_promotion_locked needs to be called.
 *          FALSE: otherwise.
 *
 * Condition: thread locked.
 */
static boolean_t
turnstile_need_thread_promotion_update(
        struct turnstile *dst_turnstile,
        thread_t thread)
{
        int thread_link_priority;
        boolean_t needs_update = FALSE;

        thread_link_priority = priority_queue_entry_key(&(dst_turnstile->ts_waitq.waitq_prio_queue),
            &(thread->wait_prioq_links));

        int priority = turnstile_compute_thread_push(dst_turnstile, thread);

        needs_update = (thread_link_priority == priority) ? FALSE : TRUE;

        return needs_update;
}

/*
 * Name: turnstile_priority_queue_update_entry_key
 *
 * Description: Updates the priority of an entry in a priority queue
 *
 * Arg1: a turnstile/thread/... priority queue
 * Arg2: the element to change the priority of
 * Arg3: the new priority
 *
 * Returns: whether the maximum priority of the queue changed.
 */
static boolean_t
turnstile_priority_queue_update_entry_key(struct priority_queue *q,
    priority_queue_entry_t elt, priority_queue_key_t pri)
{
        priority_queue_key_t old_key = priority_queue_max_key(q);

        if (priority_queue_entry_key(q, elt) < pri) {
                if (priority_queue_entry_increase(q, elt, pri,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        return old_key != priority_queue_max_key(q);
                }
        } else if (priority_queue_entry_key(q, elt) > pri) {
                if (priority_queue_entry_decrease(q, elt, pri,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        return old_key != priority_queue_max_key(q);
                }
        }

        return FALSE;
}

/*
 * Name: turnstile_update_thread_promotion_locked
 *
 * Description: Update dst turnstile's inheritor link since one of the waiting
 *              thread's priority has changed.
 *
 * Arg1: dst turnstile
 * Arg2: thread
 *
 * Returns: TRUE: if the dst turnstile priority has changed and needs propagation.
 *          FALSE: if the dst turnstile priority did not change or it does not need propagation.
 *
 * Condition: dst turnstile and thread are locked.
 */
static boolean_t
turnstile_update_thread_promotion_locked(
        struct turnstile *dst_turnstile,
        thread_t thread)
{
        int thread_link_priority;

        int priority = turnstile_compute_thread_push(dst_turnstile, thread);

        thread_link_priority = priority_queue_entry_key(&(dst_turnstile->ts_waitq.waitq_prio_queue),
            &(thread->wait_prioq_links));

        if (priority != thread_link_priority) {
                KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                    (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (THREAD_MOVED_IN_TURNSTILE_WAITQ))) | DBG_FUNC_NONE,
                    VM_KERNEL_UNSLIDE_OR_PERM(dst_turnstile),
                    thread_tid(thread),
                    priority,
                    thread_link_priority, 0);
        }

        if (!turnstile_priority_queue_update_entry_key(
                    &dst_turnstile->ts_waitq.waitq_prio_queue,
                    &thread->wait_prioq_links, priority)) {
                return FALSE;
        }

        /* Update dst turnstile's priority */
        return turnstile_recompute_priority_locked(dst_turnstile);
}

/*
 * Name: thread_add_turnstile_promotion
 *
 * Description: Add a turnstile to thread's inheritor list and update thread's priority.
 *
 * Arg1: thread
 * Arg2: turnstile
 *
 * Returns: TRUE: if the thread's priority has changed and needs propagation.
 *          FALSE: if the thread's priority did not change or it does not need propagation.
 *
 * Condition: turnstile locked.
 */
static boolean_t
thread_add_turnstile_promotion(
        thread_t thread,
        struct turnstile *turnstile)
{
        boolean_t needs_update = FALSE;

        /* Update the pairing heap */
        thread_lock(thread);

        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
            (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_ADDED_TO_THREAD_HEAP))) | DBG_FUNC_NONE,
            thread_tid(thread),
            VM_KERNEL_UNSLIDE_OR_PERM(turnstile),
            turnstile->ts_priority, 0, 0);

        priority_queue_entry_init(&turnstile->ts_inheritor_links);

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:

                if (priority_queue_insert(&(thread->base_inheritor_queue),
                    &turnstile->ts_inheritor_links, turnstile->ts_priority,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        needs_update = thread_recompute_user_promotion_locked(thread);
                }

                break;
        case TURNSTILE_KERNEL_PROMOTE:

                if (priority_queue_insert(&(thread->sched_inheritor_queue),
                    &turnstile->ts_inheritor_links, turnstile->ts_priority,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        needs_update = thread_recompute_kernel_promotion_locked(thread);
                }

                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    thread_get_update_flags_for_turnstile_propagation_stoppage(thread) |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG,
                    turnstile);
        }

        thread_unlock(thread);

        return needs_update;
}

/*
 * Name: thread_remove_turnstile_promotion
 *
 * Description: Remove turnstile from thread's inheritor list and update thread's priority.
 *
 * Arg1: thread
 * Arg2: turnstile
 *
 * Returns: TRUE: if the thread's priority has changed and needs propagation.
 *          FALSE: if the thread's priority did not change or it does not need propagation.
 *
 * Condition: turnstile locked.
 */
static boolean_t
thread_remove_turnstile_promotion(
        thread_t thread,
        struct turnstile *turnstile)
{
        boolean_t needs_update = FALSE;

        thread_lock(thread);

        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
            (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_REMOVED_FROM_THREAD_HEAP))) | DBG_FUNC_NONE,
            thread_tid(thread),
            VM_KERNEL_UNSLIDE_OR_PERM(turnstile),
            0, 0, 0);

        /* Update the pairing heap */

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
                if (priority_queue_remove(&(thread->base_inheritor_queue),
                    &turnstile->ts_inheritor_links,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        needs_update = thread_recompute_user_promotion_locked(thread);
                }
                break;
        case TURNSTILE_KERNEL_PROMOTE:
                if (priority_queue_remove(&(thread->sched_inheritor_queue),
                    &turnstile->ts_inheritor_links,
                    PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                        needs_update = thread_recompute_kernel_promotion_locked(thread);
                }
                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    thread_get_update_flags_for_turnstile_propagation_stoppage(thread) | TSU_TURNSTILE_ARG,
                    turnstile);
        }

        thread_unlock(thread);

        return needs_update;
}

/*
 * Name: thread_needs_turnstile_promotion_update
 *
 * Description: Check if turnstile position in thread's inheritor list needs to be updated.
 *
 * Arg1: thread
 * Arg2: turnstile
 *
 * Returns: TRUE: if thread_update_turnstile_promotion needs to be called.
 *          FALSE: otherwise.
 *
 * Condition: turnstile locked.
 */
static boolean_t
thread_needs_turnstile_promotion_update(
        thread_t thread __assert_only,
        struct turnstile *turnstile)
{
        boolean_t needs_update = FALSE;
        int turnstile_link_priority = 0;

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
                turnstile_link_priority = priority_queue_entry_key(&(thread->base_inheritor_queue),
                    &(turnstile->ts_inheritor_links));
                break;
        case TURNSTILE_KERNEL_PROMOTE:
                turnstile_link_priority = priority_queue_entry_key(&(thread->sched_inheritor_queue),
                    &(turnstile->ts_inheritor_links));
                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        needs_update = (turnstile_link_priority == turnstile->ts_priority) ? FALSE : TRUE;
        return needs_update;
}

/*
 * Name: thread_update_turnstile_promotion_locked
 *
 * Description: Update turnstile position in thread's inheritor list and update thread's priority.
 *
 * Arg1: thread
 * Arg2: turnstile
 *
 * Returns: TRUE: if the thread's priority has changed and needs propagation.
 *          FALSE: if the thread's priority did not change or it does not need propagation.
 *
 * Condition: turnstile and thread are locked.
 */
static boolean_t
thread_update_turnstile_promotion_locked(
        thread_t thread,
        struct turnstile *turnstile)
{
        boolean_t needs_update = FALSE;
        int turnstile_link_priority = 0;

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
                turnstile_link_priority = priority_queue_entry_key(&(thread->base_inheritor_queue), &turnstile->ts_inheritor_links);

                if (turnstile_priority_queue_update_entry_key(&(thread->base_inheritor_queue),
                    &turnstile->ts_inheritor_links, turnstile->ts_priority)) {
                        needs_update = thread_recompute_user_promotion_locked(thread);
                }
                break;
        case TURNSTILE_KERNEL_PROMOTE:
                turnstile_link_priority = priority_queue_entry_key(&(thread->sched_inheritor_queue), &turnstile->ts_inheritor_links);

                if (turnstile_priority_queue_update_entry_key(&(thread->sched_inheritor_queue),
                    &turnstile->ts_inheritor_links, turnstile->ts_priority)) {
                        needs_update = thread_recompute_kernel_promotion_locked(thread);
                }
                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        if (turnstile->ts_priority != turnstile_link_priority) {
                KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                    (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_MOVED_IN_THREAD_HEAP))) | DBG_FUNC_NONE,
                    thread_tid(thread),
                    VM_KERNEL_UNSLIDE_OR_PERM(turnstile),
                    turnstile->ts_priority,
                    turnstile_link_priority, 0);
        }

        return needs_update;
}


/*
 * Name: thread_update_turnstile_promotion
 *
 * Description: Update turnstile position in thread's inheritor list and update thread's priority.
 *
 * Arg1: thread
 * Arg2: turnstile
 *
 * Returns: TRUE: if the thread's priority has changed and needs propagation.
 *          FALSE: if the thread's priority did not change or it does not need propagation.
 *
 * Condition: turnstile locked.
 */
static boolean_t
thread_update_turnstile_promotion(
        thread_t thread,
        struct turnstile *turnstile)
{
        /* Before grabbing the thread lock, check if update is needed */
        boolean_t needs_update = thread_needs_turnstile_promotion_update(thread, turnstile);

        if (!needs_update) {
                turnstile_stats_update(1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG, turnstile);
                return needs_update;
        }

        thread_lock(thread);

        /* Update the pairing heap */
        needs_update = thread_update_turnstile_promotion_locked(thread, turnstile);

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    thread_get_update_flags_for_turnstile_propagation_stoppage(thread) |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG,
                    turnstile);
        }

        thread_unlock(thread);

        return needs_update;
}


/*
 * Name: thread_get_inheritor_turnstile_sched_priority
 *
 * Description: Get the max sched priority of all the inheritor turnstiles
 *
 * Arg1: thread
 *
 * Returns: Max sched priority of all the inheritor turnstiles.
 *
 * Condition: thread locked
 */
int
thread_get_inheritor_turnstile_sched_priority(thread_t thread)
{
        struct turnstile *max_turnstile;

        max_turnstile = priority_queue_max(&thread->sched_inheritor_queue,
            struct turnstile, ts_inheritor_links);

        if (max_turnstile) {
                return priority_queue_entry_key(&thread->sched_inheritor_queue,
                           &max_turnstile->ts_inheritor_links);
        }

        return 0;
}

/*
 * Name: thread_get_inheritor_turnstile_base_priority
 *
 * Description: Get the max base priority of all the inheritor turnstiles
 *
 * Arg1: thread
 *
 * Returns: Max base priority of all the inheritor turnstiles.
 *
 * Condition: thread locked
 */
int
thread_get_inheritor_turnstile_base_priority(thread_t thread)
{
        struct turnstile *max_turnstile;

        max_turnstile = priority_queue_max(&thread->base_inheritor_queue,
            struct turnstile, ts_inheritor_links);

        if (max_turnstile) {
                return priority_queue_entry_key(&thread->base_inheritor_queue,
                           &max_turnstile->ts_inheritor_links);
        }

        return 0;
}


/*
 * Name: thread_get_waiting_turnstile
 *
 * Description: Get the turnstile if the thread is waiting on a turnstile.
 *
 * Arg1: thread
 *
 * Returns: turnstile: if the thread is blocked on a turnstile.
 *          TURNSTILE_NULL: otherwise.
 *
 * Condition: thread locked.
 */
struct turnstile *
thread_get_waiting_turnstile(thread_t thread)
{
        struct turnstile *turnstile = TURNSTILE_NULL;
        struct waitq *waitq = thread->waitq;

        /* Check if the thread is on a waitq */
        if (waitq == NULL) {
                return turnstile;
        }

        /* Get the safeq if the waitq is a port queue */
        if (waitq_is_port_queue(waitq)) {
                waitq = waitq_get_safeq(waitq);
        }

        /* Check if the waitq is a turnstile queue */
        if (waitq_is_turnstile_queue(waitq)) {
                turnstile = waitq_to_turnstile(waitq);
        }
        return turnstile;
}

/*
 * Name: turnstile_lookup_by_proprietor
 *
 * Description: Get turnstile for a proprietor from global
 *              turnstile hash.
 *
 * Arg1: port
 * Arg2: turnstile_type_t type
 *
 * Returns: turnstile: if the proprietor has a turnstile.
 *          TURNSTILE_NULL: otherwise.
 *
 * Condition: proprietor interlock held.
 */
struct turnstile *
turnstile_lookup_by_proprietor(uintptr_t proprietor, turnstile_type_t type)
{
        return turnstile_htable_lookup(proprietor, type);
}

/*
 * Name: thread_get_update_flags_for_turnstile_propagation_stoppage
 *
 * Description: Get the turnstile stats flags based on the thread wait status.
 *
 * Arg1: thread
 *
 * Returns: TSU_THREAD_RUNNABLE: if the thread is runnable.
 *          TSU_NO_TURNSTILE: if thread waiting on a regular waitq.
 *          TSU_NO_PRI_CHANGE_NEEDED: otherwise.
 *
 * Condition: thread locked.
 */
static turnstile_stats_update_flags_t
thread_get_update_flags_for_turnstile_propagation_stoppage(thread_t thread)
{
        struct waitq *waitq = thread->waitq;

        /* Check if the thread is on a waitq */
        if (waitq == NULL) {
                return TSU_THREAD_RUNNABLE;
        }

        /* Get the safeq if the waitq is a port queue */
        if (waitq_is_port_queue(waitq)) {
                waitq = waitq_get_safeq(waitq);
        }

        /* Check if the waitq is a turnstile queue */
        if (!waitq_is_turnstile_queue(waitq)) {
                return TSU_NO_TURNSTILE;
        }

        /* Thread blocked on turnstile waitq but no propagation needed */
        return TSU_NO_PRI_CHANGE_NEEDED;
}


/*
 * Name: turnstile_get_update_flags_for_above_UI_pri_change
 *
 * Description: Get the turnstile stats flags based on the turnstile priority.
 *
 * Arg1: turnstile
 *
 * Returns: TSU_ABOVE_UI_PRI_CHANGE: if turnstile priority is above 47 and it is not an ulock.
 *          TSU_FLAGS_NONE: otherwise.
 *
 * Condition: turnstile locked.
 */
static turnstile_stats_update_flags_t
turnstile_get_update_flags_for_above_UI_pri_change(struct turnstile *turnstile)
{
        if (turnstile->ts_priority >
            (thread_qos_policy_params.qos_pri[THREAD_QOS_USER_INTERACTIVE] + 1) &&
            turnstile_get_type(turnstile) != TURNSTILE_ULOCK) {
                return TSU_ABOVE_UI_PRI_CHANGE;
        }

        return TSU_FLAGS_NONE;
}


/*
 * Name: workq_add_turnstile_promotion
 *
 * Description: Connect the workqueue turnstile to the workqueue as a fake
 *              inheritor
 *
 * Arg1: workqueue
 * Arg2: turnstile
 *
 * Condition: turnstile locked.
 */
static boolean_t
workq_add_turnstile_promotion(
        struct workqueue *wq_inheritor __unused,
        struct turnstile *turnstile)
{
        /*
         * If the push is higher than MAXPRI_THROTTLE then the workqueue should
         * bring up a thread.
         */
        return turnstile->ts_priority > MAXPRI_THROTTLE;
}

/*
 * Name: turnstile_need_turnstile_promotion_update
 *
 * Description: Check if turnstile position in turnstile's inheritor list needs to be updated.
 *
 * Arg1: dst turnstile
 * Arg2: src turnstile
 *
 * Returns: TRUE: if turnstile_update_turnstile_promotion needs to be called.
 *          FALSE: otherwise.
 *
 * Condition: src turnstile locked.
 */
static boolean_t
turnstile_need_turnstile_promotion_update(
        struct turnstile *dst_turnstile __assert_only,
        struct turnstile *src_turnstile)
{
        int src_turnstile_link_priority;
        boolean_t needs_update = FALSE;

        src_turnstile_link_priority = priority_queue_entry_key(&(dst_turnstile->ts_inheritor_queue),
            &(src_turnstile->ts_inheritor_links));

        needs_update = (src_turnstile_link_priority == src_turnstile->ts_priority) ? FALSE : TRUE;
        return needs_update;
}

/*
 * Name: turnstile_update_turnstile_promotion_locked
 *
 * Description: Update dst turnstile's inheritor link since src turnstile's
 *              promote priority has changed.
 *
 * Arg1: dst turnstile
 * Arg2: src turnstile
 *
 * Returns: TRUE: if the dst turnstile priority has changed and needs propagation.
 *          FALSE: if the dst turnstile priority did not change or it does not need propagation.
 *
 * Condition: src and dst turnstile locked.
 */
static boolean_t
turnstile_update_turnstile_promotion_locked(
        struct turnstile *dst_turnstile,
        struct turnstile *src_turnstile)
{
        int src_turnstile_link_priority;
        src_turnstile_link_priority = priority_queue_entry_key(&(dst_turnstile->ts_inheritor_queue),
            &(src_turnstile->ts_inheritor_links));

        if (src_turnstile->ts_priority != src_turnstile_link_priority) {
                KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                    (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_MOVED_IN_TURNSTILE_HEAP))) | DBG_FUNC_NONE,
                    VM_KERNEL_UNSLIDE_OR_PERM(dst_turnstile),
                    VM_KERNEL_UNSLIDE_OR_PERM(src_turnstile),
                    src_turnstile->ts_priority, src_turnstile_link_priority, 0);
        }

        if (!turnstile_priority_queue_update_entry_key(
                    &dst_turnstile->ts_inheritor_queue, &src_turnstile->ts_inheritor_links,
                    src_turnstile->ts_priority)) {
                return FALSE;
        }

        /* Update dst turnstile's priority */
        return turnstile_recompute_priority_locked(dst_turnstile);
}

/*
 * Name: turnstile_update_turnstile_promotion
 *
 * Description: Update dst turnstile's inheritor link since src turnstile's
 *              promote priority has changed.
 *
 * Arg1: dst turnstile
 * Arg2: src turnstile
 *
 * Returns: TRUE: if the dst turnstile priority has changed and needs propagation.
 *          FALSE: if the dst turnstile priority did not change or it does not need propagation.
 *
 * Condition: src turnstile locked.
 */
static boolean_t
turnstile_update_turnstile_promotion(
        struct turnstile *dst_turnstile,
        struct turnstile *src_turnstile)
{
        /* Check if update is needed before grabbing the src turnstile lock */
        boolean_t needs_update = turnstile_need_turnstile_promotion_update(dst_turnstile, src_turnstile);
        if (!needs_update) {
                turnstile_stats_update(1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG,
                    src_turnstile);
                return needs_update;
        }

        /* Update the pairing heap */
        waitq_lock(&dst_turnstile->ts_waitq);
        needs_update = turnstile_update_turnstile_promotion_locked(dst_turnstile, src_turnstile);

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    (dst_turnstile->ts_inheritor ? TSU_NO_PRI_CHANGE_NEEDED : TSU_NO_INHERITOR) |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG, src_turnstile);
        }
        waitq_unlock(&dst_turnstile->ts_waitq);
        return needs_update;
}

/*
 * Name: turnstile_add_turnstile_promotion
 *
 * Description: Add src turnstile to dst turnstile's inheritor link
 *              and update dst turnstile's priority.
 *
 * Arg1: dst turnstile
 * Arg2: src turnstile
 *
 * Returns: TRUE: if the dst turnstile priority has changed and needs propagation.
 *          FALSE: if the dst turnstile priority did not change or it does not need propagation.
 *
 * Condition: src turnstile locked.
 */
static boolean_t
turnstile_add_turnstile_promotion(
        struct turnstile *dst_turnstile,
        struct turnstile *src_turnstile)
{
        boolean_t needs_update = FALSE;

        /* Update the pairing heap */
        waitq_lock(&dst_turnstile->ts_waitq);

        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
            (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_ADDED_TO_TURNSTILE_HEAP))) | DBG_FUNC_NONE,
            VM_KERNEL_UNSLIDE_OR_PERM(dst_turnstile),
            VM_KERNEL_UNSLIDE_OR_PERM(src_turnstile),
            src_turnstile->ts_priority, 0, 0);

        priority_queue_entry_init(&(src_turnstile->ts_inheritor_links));
        if (priority_queue_insert(&dst_turnstile->ts_inheritor_queue,
            &src_turnstile->ts_inheritor_links, src_turnstile->ts_priority,
            PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                /* Update dst turnstile priority */
                needs_update = turnstile_recompute_priority_locked(dst_turnstile);
        }

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    (dst_turnstile->ts_inheritor ? TSU_NO_PRI_CHANGE_NEEDED : TSU_NO_INHERITOR) |
                    TSU_TURNSTILE_ARG | TSU_BOOST_ARG, src_turnstile);
        }

        waitq_unlock(&dst_turnstile->ts_waitq);
        return needs_update;
}

/*
 * Name: turnstile_remove_turnstile_promotion
 *
 * Description: Remove src turnstile from dst turnstile's inheritor link
 *              and update dst turnstile's priority.
 *
 * Arg1: dst turnstile
 * Arg2: src turnstile
 *
 * Returns: TRUE: if the dst turnstile priority has changed and needs propagation.
 *          FALSE: if the dst turnstile priority did not change or it does not need propagation.
 *
 * Condition: src turnstile locked.
 */
static boolean_t
turnstile_remove_turnstile_promotion(
        struct turnstile *dst_turnstile,
        struct turnstile *src_turnstile)
{
        boolean_t needs_update = FALSE;

        /* Update the pairing heap */
        waitq_lock(&dst_turnstile->ts_waitq);

        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
            (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (TURNSTILE_REMOVED_FROM_TURNSTILE_HEAP))) | DBG_FUNC_NONE,
            VM_KERNEL_UNSLIDE_OR_PERM(dst_turnstile),
            VM_KERNEL_UNSLIDE_OR_PERM(src_turnstile),
            0, 0, 0);

        if (priority_queue_remove(&dst_turnstile->ts_inheritor_queue,
            &src_turnstile->ts_inheritor_links,
            PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE)) {
                /* Update dst turnstile priority */
                needs_update = turnstile_recompute_priority_locked(dst_turnstile);
        }

        /* Update turnstile stats */
        if (!needs_update) {
                turnstile_stats_update(1,
                    (dst_turnstile->ts_inheritor ? TSU_NO_PRI_CHANGE_NEEDED : TSU_NO_INHERITOR) |
                    TSU_TURNSTILE_ARG, src_turnstile);
        }

        waitq_unlock(&dst_turnstile->ts_waitq);
        return needs_update;
}

/*
 * Name: turnstile_compute_thread_push
 *
 * Description: Compute the priority at which the thread will push
 *       on the turnstile.
 *
 * Arg1: turnstile
 * Arg2: thread
 *
 * Condition: wq locked
 */
static int
turnstile_compute_thread_push(
        struct turnstile *turnstile,
        thread_t thread)
{
        int priority = 0;
        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
                priority = thread->base_pri;
                break;
        case TURNSTILE_KERNEL_PROMOTE:
                /*
                 * Ideally this should be policy based
                 * according to the turnstile type.
                 *
                 * The priority with which each thread pushes on
                 * a primitive should be primitive dependent.
                 */
                priority = thread->sched_pri;
                priority = MAX(priority, thread->base_pri);
                priority = MAX(priority, BASEPRI_DEFAULT);
                priority = MIN(priority, MAXPRI_PROMOTE);
                break;
        default:
                panic("turnstile promotion for type %d not yet implemented", turnstile_get_type(turnstile));
        }

        return priority;
}

/*
 * Name: turnstile_waitq_add_thread_priority_queue
 *
 * Description: add thread to the turnstile wq
 *
 * Arg1: turnstile wq
 * Arg2: thread to add
 *
 * Condition: wq locked
 */
void
turnstile_waitq_add_thread_priority_queue(
        struct waitq *wq,
        thread_t thread)
{
        struct turnstile *turnstile = waitq_to_turnstile(wq);
        int priority = turnstile_compute_thread_push(turnstile, thread);

        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
            (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS, (THREAD_ADDED_TO_TURNSTILE_WAITQ))) | DBG_FUNC_NONE,
            VM_KERNEL_UNSLIDE_OR_PERM(turnstile),
            thread_tid(thread),
            priority, 0, 0);
        /*
         * For turnstile queues (which use priority queues),
         * insert the thread in the heap based on its priority.
         * Note that the priority queue implementation
         * is currently not stable, so does not maintain fifo for
         * threads at the same pri. Also, if the pri
         * of the thread changes while its blocked in the waitq,
         * the thread position should be updated in the priority
         * queue by calling priority queue increase/decrease
         * operations.
         */
        priority_queue_entry_init(&(thread->wait_prioq_links));
        priority_queue_insert(&wq->waitq_prio_queue,
            &thread->wait_prioq_links, priority,
            PRIORITY_QUEUE_SCHED_PRI_MAX_HEAP_COMPARE);
}

/*
 * Name: turnstile_recompute_priority_locked
 *
 * Description: Update turnstile priority based
 *              on highest waiter thread and highest blocking
 *              turnstile.
 *
 * Args: turnstile
 *
 * Returns: TRUE: if the turnstile priority changed and needs propagation.
 *          FALSE: if the turnstile priority did not change or it does not need propagation.
 *
 * Condition: turnstile locked
 */
boolean_t
turnstile_recompute_priority_locked(
        struct turnstile *turnstile)
{
        int old_priority;
        int new_priority;
        boolean_t needs_priority_update = FALSE;
        thread_t max_thread = THREAD_NULL;
        struct turnstile *max_turnstile;
        int thread_max_pri = 0;
        int turnstile_max_pri = 0;

        switch (turnstile_promote_policy[turnstile_get_type(turnstile)]) {
        case TURNSTILE_USER_PROMOTE:
        case TURNSTILE_USER_IPC_PROMOTE:
        case TURNSTILE_KERNEL_PROMOTE:

                old_priority = turnstile->ts_priority;

                max_thread = priority_queue_max(&turnstile->ts_waitq.waitq_prio_queue,
                    struct thread, wait_prioq_links);

                if (max_thread) {
                        thread_max_pri = priority_queue_entry_key(&turnstile->ts_waitq.waitq_prio_queue,
                            &max_thread->wait_prioq_links);
                }

                max_turnstile = priority_queue_max(&turnstile->ts_inheritor_queue,
                    struct turnstile, ts_inheritor_links);

                if (max_turnstile) {
                        assert(turnstile_promote_policy[turnstile_get_type(turnstile)] != TURNSTILE_KERNEL_PROMOTE);
                        turnstile_max_pri = priority_queue_entry_key(&turnstile->ts_inheritor_queue,
                            &max_turnstile->ts_inheritor_links);
                }

                new_priority = max(thread_max_pri, turnstile_max_pri);
                turnstile->ts_priority = new_priority;

                if (old_priority != new_priority) {
                        KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                            (TURNSTILE_CODE(TURNSTILE_PRIORITY_OPERATIONS,
                            (TURNSTILE_PRIORITY_CHANGE))) | DBG_FUNC_NONE,
                            VM_KERNEL_UNSLIDE_OR_PERM(turnstile),
                            new_priority,
                            old_priority,
                            0, 0);
                }
                needs_priority_update = (!(old_priority == new_priority)) &&
                    (turnstile->ts_inheritor != NULL);
                break;

        case TURNSTILE_PROMOTE_NONE:
                /* The turnstile was repurposed, do nothing */
                break;

        default:

                panic("Needs implementation for turnstile_recompute_priority");
                break;
        }
        return needs_priority_update;
}


/*
 * Name: turnstile_recompute_priority
 *
 * Description: Update turnstile priority based
 *              on highest waiter thread and highest blocking
 *              turnstile.
 *
 * Args: turnstile
 *
 * Returns: TRUE: if the turnstile priority changed and needs propagation.
 *          FALSE: if the turnstile priority did not change or it does not need propagation.
 */
boolean_t
turnstile_recompute_priority(
        struct turnstile *turnstile)
{
        boolean_t needs_priority_update = FALSE;
        spl_t s = splsched();

        waitq_lock(&turnstile->ts_waitq);

        needs_priority_update = turnstile_recompute_priority_locked(turnstile);

        waitq_unlock(&turnstile->ts_waitq);
        splx(s);
        return needs_priority_update;
}


/*
 * Name: turnstile_workq_proprietor_of_max_turnstile
 *
 * Description: Returns the highest priority and proprietor of a turnstile
 *              pushing on a workqueue turnstile.
 *
 *              This will not return waiters that are at priority
 *              MAXPRI_THROTTLE or lower.
 *
 * Args: turnstile
 *
 * Returns:
 *    Priority of the max entry, or 0
 *    Pointer to the max entry proprietor
 */
int
turnstile_workq_proprietor_of_max_turnstile(
        struct turnstile *turnstile,
        uintptr_t *proprietor_out)
{
        struct turnstile *max_turnstile;
        int max_priority = 0;
        uintptr_t proprietor = 0;

        assert(turnstile_get_type(turnstile) == TURNSTILE_WORKQS);

        spl_t s = splsched();

        waitq_lock(&turnstile->ts_waitq);

        max_turnstile = priority_queue_max(&turnstile->ts_inheritor_queue,
            struct turnstile, ts_inheritor_links);
        if (max_turnstile) {
                max_priority = priority_queue_entry_key(&turnstile->ts_inheritor_queue,
                    &max_turnstile->ts_inheritor_links);
                proprietor = max_turnstile->ts_proprietor;
        }

        waitq_unlock(&turnstile->ts_waitq);
        splx(s);

        if (max_priority <= MAXPRI_THROTTLE) {
                max_priority = 0;
                proprietor = 0;
        }
        if (proprietor_out) {
                *proprietor_out = proprietor;
        }
        return max_priority;
}

/*
 * Name: turnstile_workloop_pusher_info
 *
 * Description: Returns the priority of the turnstile push for a workloop,
 *              and the thread or knote responsible for this push.
 *
 * Args: workloop turnstile
 *
 * Returns:
 *    Priority of the push or 0
 *    Thread (with a +1 reference) with that push or THREAD_NULL.
 *    Port (with a +1 reference) with that push, or IP_NULL.
 *    Sync IPC knote with the highest push (or NULL)
 */
int
turnstile_workloop_pusher_info(
        struct turnstile *turnstile,
        thread_t *thread_out,
        ipc_port_t *port_out,
        struct knote **knote_out)
{
        struct turnstile *max_ts;
        thread_t max_thread;
        int max_thread_pri = 0;
        int max_ts_pri = 0;
        ipc_port_t port;

        assert(turnstile_get_type(turnstile) == TURNSTILE_WORKLOOPS);

        spl_t s = splsched();
        waitq_lock(&turnstile->ts_waitq);

        max_thread = priority_queue_max(&turnstile->ts_waitq.waitq_prio_queue,
            struct thread, wait_prioq_links);
        if (max_thread) {
                max_thread_pri = priority_queue_entry_key(
                        &turnstile->ts_waitq.waitq_prio_queue,
                        &max_thread->wait_prioq_links);
        }

        max_ts = priority_queue_max(&turnstile->ts_inheritor_queue,
            struct turnstile, ts_inheritor_links);
        if (max_ts) {
                max_ts_pri = priority_queue_entry_key(&turnstile->ts_inheritor_queue,
                    &max_ts->ts_inheritor_links);
        }

        /*
         * Reasons to push on a workloop turnstile are:
         *
         * 1. threads in dispatch sync
         *
         * 2. sync IPC pushes, which in turn have 4 sub-cases:
         *
         *   2.a. special reply port or receive right pushing through a knote
         *        turnstile,
         *
         *   2.b. special reply port stashed on a knote, pushing on the workloop
         *        directly,
         *
         *   2.c. receive right stashed on a knote, pushing on the workloop
         *        directly,
         *
         *   2.d. a receive right monitored by a knote, pushing on the workloop
         *        directly.
         *
         * See ipc_port_send_update_inheritor(), ipc_port_recv_update_inheritor().
         *
         * Note: dereferencing the knote in the caller is safe provided this
         * function i scalled under the proper interlocks (the filt_wllock + req
         * lock) which serializes with the knote going away.
         */
        if (max_thread_pri > max_ts_pri) {
                thread_reference(max_thread);
                *thread_out = max_thread;
                *port_out = NULL;
                *knote_out = NULL;
        } else if (max_ts_pri) {
                switch (turnstile_get_type(max_ts)) {
                case TURNSTILE_KNOTE:
                        /* 2.a. */
                        *thread_out = THREAD_NULL;
                        *port_out = IP_NULL;
                        *knote_out = (struct knote *)max_ts->ts_proprietor;
                        break;

                case TURNSTILE_SYNC_IPC:
                        /* 2.[bcd] */
                        port = (ipc_port_t)max_ts->ts_proprietor;
                        ip_reference(port);
                        *thread_out = THREAD_NULL;
                        *port_out = port;
                        *knote_out = NULL;
                        break;

                default:
                        panic("Unexpected type for turnstile %p", max_ts);
                }
        } else {
                *thread_out = THREAD_NULL;
                *port_out = IP_NULL;
                *knote_out = NULL;
        }

        waitq_unlock(&turnstile->ts_waitq);
        splx(s);

        return max(max_thread_pri, max_ts_pri);
}

/*
 * Name: turnstile_has_waiters
 *
 * Description: returns if there are waiters on the turnstile
 *
 * Arg1: turnstile: turnstile
 *
 * Returns: TRUE if there are waiters, FALSE otherwise.
 */

boolean_t
turnstile_has_waiters(struct turnstile *turnstile)
{
        boolean_t ret;

        spl_t s = splsched();
        waitq_lock(&turnstile->ts_waitq);
        ret = !priority_queue_empty(&turnstile->ts_waitq.waitq_prio_queue);
        waitq_unlock(&turnstile->ts_waitq);
        splx(s);

        return ret;
}

/*
 * Name: turnstile_update_inheritor_priority_chain
 *
 * Description: Update turnstile inheritor's priority and propagate
 *              the priority if the inheritor is blocked on a turnstile.
 *
 * Arg1: inheritor
 * Arg2: inheritor flags
 *
 * Returns: None.
 */
static void
turnstile_update_inheritor_priority_chain(
        turnstile_inheritor_t inheritor,
        turnstile_update_flags_t turnstile_flags)
{
        struct turnstile *turnstile = TURNSTILE_NULL;
        thread_t thread = THREAD_NULL;
        int total_hop = 0, thread_hop = 0;
        spl_t s;
        turnstile_stats_update_flags_t tsu_flags = ((turnstile_flags & TURNSTILE_UPDATE_BOOST) ?
            TSU_BOOST_ARG : TSU_FLAGS_NONE) | TSU_PRI_PROPAGATION;

        if (inheritor == NULL) {
                return;
        }

        s = splsched();

        if (turnstile_flags & TURNSTILE_INHERITOR_THREAD) {
                thread = inheritor;
                thread_lock(thread);
                thread_recompute_user_promotion_locked(thread);
                thread_recompute_kernel_promotion_locked(thread);
        } else if (turnstile_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                turnstile = inheritor;
                waitq_lock(&turnstile->ts_waitq);
                turnstile_recompute_priority_locked(turnstile);
                tsu_flags |= turnstile_get_update_flags_for_above_UI_pri_change(turnstile);
        } else {
                /*
                 * we should never call turnstile_update_inheritor_priority_chain()
                 * for a workqueue, they have no "chain" after them.
                 */
                assert((turnstile_flags & TURNSTILE_INHERITOR_WORKQ) == 0);
        }

        while (turnstile != TURNSTILE_NULL || thread != THREAD_NULL) {
                if (turnstile != TURNSTILE_NULL) {
                        if (turnstile->ts_inheritor == NULL) {
                                turnstile_stats_update(total_hop + 1, TSU_NO_INHERITOR |
                                    TSU_TURNSTILE_ARG | tsu_flags,
                                    turnstile);
                                waitq_unlock(&turnstile->ts_waitq);
                                turnstile = TURNSTILE_NULL;
                                break;
                        }
                        if (turnstile->ts_inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                                turnstile_update_inheritor_thread_priority_chain(&turnstile, &thread,
                                    total_hop, tsu_flags);
                        } else if (turnstile->ts_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                                turnstile_update_inheritor_turnstile_priority_chain(&turnstile,
                                    total_hop, tsu_flags);
                        } else if (turnstile->ts_inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                                turnstile_update_inheritor_workq_priority_chain(turnstile, s);
                                turnstile_stats_update(total_hop + 1, TSU_NO_PRI_CHANGE_NEEDED | tsu_flags,
                                    NULL);
                                return;
                        } else {
                                panic("Inheritor flags not passed in turnstile_update_inheritor");
                        }
                } else if (thread != THREAD_NULL) {
                        thread_update_waiting_turnstile_priority_chain(&thread, &turnstile,
                            thread_hop, total_hop, tsu_flags);
                        thread_hop++;
                }
                total_hop++;
        }

        splx(s);
        return;
}

/*
 * Name: turnstile_update_inheritor_complete
 *
 * Description: Update turnstile inheritor's priority and propagate the
 *              priority if the inheritor is blocked on a turnstile.
 *              Consumes thread ref of old inheritor returned by
 *              turnstile_update_inheritor. Recursive priority update
 *              will only happen when called with interlock dropped.
 *
 * Args:
 *   Arg1: turnstile
 *   Arg2: interlock held
 *
 * Returns: None.
 */
void
turnstile_update_inheritor_complete(
        struct turnstile *turnstile,
        turnstile_update_complete_flags_t flags __unused)
{
        thread_t thread = current_thread();

        turnstile_update_flags_t inheritor_flags = thread->inheritor_flags;

        turnstile_cleanup();

        /* Perform priority update for new inheritor */
        if (inheritor_flags & TURNSTILE_NEEDS_PRI_UPDATE) {
                turnstile_update_inheritor_priority_chain(turnstile,
                    TURNSTILE_INHERITOR_TURNSTILE | TURNSTILE_UPDATE_BOOST);
        }
}

/*
 * Name: turnstile_cleanup
 *
 * Description: Update priority of a turnstile inheritor
 *              if needed.
 *
 * Args: inheritor and flags passed on thread struct.
 *
 * Returns: None.
 */
void
turnstile_cleanup(void)
{
        thread_t thread = current_thread();

        /* Get the old inheritor from calling thread struct */
        turnstile_inheritor_t old_inheritor = thread->inheritor;
        turnstile_update_flags_t inheritor_flags = thread->inheritor_flags;
        thread->inheritor = THREAD_NULL;
        thread->inheritor_flags = TURNSTILE_UPDATE_FLAGS_NONE;

        if (old_inheritor == TURNSTILE_INHERITOR_NULL) {
                /* no cleanup to do */
                return;
        }

        /* Perform priority demotion for old inheritor */
        if (inheritor_flags & TURNSTILE_INHERITOR_NEEDS_PRI_UPDATE) {
                turnstile_update_inheritor_priority_chain(old_inheritor,
                    inheritor_flags);
        }

        /* Drop thread reference for old inheritor */
        if (inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                thread_deallocate_safe(old_inheritor);
        } else if (inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                turnstile_deallocate_safe((struct turnstile *)old_inheritor);
        } else if (inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                workq_deallocate_safe((struct workqueue *)old_inheritor);
        } else {
                panic("Inheritor flags lost along the way");
        }
}

/*
 * Name: turnstile_update_thread_priority_chain
 *
 * Description: Priority of a thread blocked on a turnstile
 *              has changed, update the turnstile priority.
 *
 * Arg1: thread: thread whose priority has changed.
 *
 * Returns: None.
 */
void
turnstile_update_thread_priority_chain(thread_t thread)
{
        turnstile_update_inheritor_priority_chain(thread,
            TURNSTILE_INHERITOR_THREAD | TURNSTILE_UPDATE_BOOST);
}

/*
 * Name: turnstile_update_inheritor_workq_priority_chain
 *
 * Description: Helper function to update turnstile's inheritor(workq)
 *              priority and possibly redrive thread creation
 *
 * Arg1: turnstile: turnstile
 * Arg2: s: whether iterrupts are disabled.
 *
 * Condition: turnstile is locked on entry, it is unlocked on exit,
 *            and interrupts re-enabled.
 */
static void
turnstile_update_inheritor_workq_priority_chain(struct turnstile *turnstile, spl_t s)
{
        struct workqueue *wq = turnstile->ts_inheritor;
        bool workq_lock_held = workq_is_current_thread_updating_turnstile(wq);

        if (__improbable(turnstile->ts_priority <= MAXPRI_THROTTLE)) {
                waitq_unlock(&turnstile->ts_waitq);
                splx(s);
                return;
        }

        if (!workq_lock_held) {
                workq_reference(wq);
                disable_preemption();
        }
        waitq_unlock(&turnstile->ts_waitq);
        splx(s);

        workq_schedule_creator_turnstile_redrive(wq, workq_lock_held);

        if (!workq_lock_held) {
                enable_preemption();
                workq_deallocate_safe(wq);
        }
}

/*
 * Name: turnstile_update_inheritor_thread_priority_chain
 *
 * Description: Helper function to update turnstile's inheritor(thread)
 *              priority.
 *
 * Arg1: in_turnstile: address to turnstile
 * Arg2: out_thread: address to return the thread inheritor
 * Arg3: thread_hop: number to thread hop in propagation chain
 * Arg4: tsu_flags: turnstile update flags
 *
 * Returns: Implicit returns locked thread in out_thread if it needs
 *          further propagation.
 *
 * Condition: *in_turnstile is locked on entry, it is unlocked on exit and
 *            *in_turnstile is set to NULL.
 */
static void
turnstile_update_inheritor_thread_priority_chain(
        struct turnstile **in_turnstile,
        thread_t *out_thread,
        int total_hop,
        turnstile_stats_update_flags_t tsu_flags)
{
        boolean_t needs_update = FALSE;
        struct turnstile *turnstile = *in_turnstile;
        thread_t thread_inheritor = turnstile->ts_inheritor;
        boolean_t first_update = !total_hop;

        assert(turnstile->ts_inheritor_flags & TURNSTILE_INHERITOR_THREAD);
        *in_turnstile = TURNSTILE_NULL;

        /* Check if update is needed before grabbing the thread lock */
        needs_update = thread_needs_turnstile_promotion_update(thread_inheritor, turnstile);
        if (!needs_update && !first_update) {
                turnstile_stats_update(total_hop + 1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_TURNSTILE_ARG | tsu_flags, turnstile);
                waitq_unlock(&turnstile->ts_waitq);
                return;
        }

        thread_lock(thread_inheritor);

        /* adjust turnstile position in the thread's inheritor list */
        needs_update = thread_update_turnstile_promotion_locked(
                thread_inheritor, turnstile);

        /*
         * Check if thread needs further priority propagation,
         * since the first hop priority update was done in
         * turnstile_update_inheritor, do not bailout if it is
         * the first update as needs_update flag would evaluate to
         * false for that case.
         */
        if (!needs_update && !first_update) {
                /* Update turnstile stats before returning */
                turnstile_stats_update(total_hop + 1,
                    (thread_get_update_flags_for_turnstile_propagation_stoppage(thread_inheritor)) |
                    TSU_TURNSTILE_ARG | tsu_flags,
                    turnstile);
                thread_unlock(thread_inheritor);
                waitq_unlock(&turnstile->ts_waitq);
                return;
        }

        /* Unlock the turnstile and update the thread */
        waitq_unlock(&turnstile->ts_waitq);
        *out_thread = thread_inheritor;
        return;
}

/*
 * Name: turnstile_update_inheritor_turnstile_priority_chain
 *
 * Description: Helper function to update turnstile's inheritor(turnstile)
 *              priority.
 *
 * Arg1: in_out_turnstile: address to turnstile
 * Arg2: thread_hop: number of thread hop in propagation chain
 * Arg3: tsu_flags: turnstile update flags
 *
 * Returns: Implicit returns locked turnstile in in_out_turnstile if it needs
 *          further propagation.
 *
 * Condition: *in_out_turnstile is locked on entry, *in_out_turnstile on exit,
 *            but the value of *in_out_turnstile might change and turnstile lock
 *            will be dropped for old value and will be acquired for the new value.
 */
static void
turnstile_update_inheritor_turnstile_priority_chain(
        struct turnstile **in_out_turnstile,
        int total_hop,
        turnstile_stats_update_flags_t tsu_flags)
{
        boolean_t needs_update = FALSE;
        struct turnstile *turnstile = *in_out_turnstile;
        struct turnstile *inheritor_turnstile = turnstile->ts_inheritor;
        boolean_t first_update = !total_hop;

        assert(turnstile->ts_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE);
        *in_out_turnstile = TURNSTILE_NULL;

        /* Check if the inheritor turnstile needs to be updated before grabbing the lock */
        needs_update = turnstile_need_turnstile_promotion_update(inheritor_turnstile, turnstile);
        if (!needs_update && !first_update) {
                turnstile_stats_update(total_hop + 1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_TURNSTILE_ARG | tsu_flags,
                    turnstile);
                waitq_unlock(&turnstile->ts_waitq);
                return;
        }

        waitq_lock(&inheritor_turnstile->ts_waitq);

        needs_update = turnstile_update_turnstile_promotion_locked(
                inheritor_turnstile, turnstile);

        /*
         * Check if turnstile needs further priority propagation,
         * since the first hop priority update was done in
         * turnstile_update_inheritor, do not bailout if it is
         * the first update as needs_update flag would evaluate to
         * false for that case.
         */
        if (!needs_update && !first_update) {
                /* Update turnstile stats before returning */
                turnstile_stats_update(total_hop + 1,
                    (inheritor_turnstile->ts_inheritor ? TSU_NO_PRI_CHANGE_NEEDED : TSU_NO_INHERITOR) |
                    TSU_TURNSTILE_ARG | tsu_flags,
                    turnstile);
                waitq_unlock(&inheritor_turnstile->ts_waitq);
                waitq_unlock(&turnstile->ts_waitq);
                return;
        }

        /* Unlock the outer turnstile and update the inner turnstile */
        waitq_unlock(&turnstile->ts_waitq);
        *in_out_turnstile = inheritor_turnstile;
        return;
}

/*
 * Name: thread_update_waiting_turnstile_priority_chain
 *
 * Description: Helper function to update thread's waiting
 *              turnstile priority.
 *
 * Arg1: in_thread: pointer to thread
 * Arg2: out_turnstile: pointer to turnstile to return to caller
 * Arg3: thread_hop: Number of thread hops visited
 * Arg4: total_hop: total hops visited
 * Arg5: tsu_flags: turnstile update flags
 *
 * Returns: *out_turnstile returns the inheritor if it needs further propagation.
 *
 * Condition: *in_thread locked on entry, unlocked on exit and set to NULL.
 */
static void
thread_update_waiting_turnstile_priority_chain(
        thread_t *in_thread,
        struct turnstile **out_turnstile,
        int thread_hop,
        int total_hop,
        turnstile_stats_update_flags_t tsu_flags)
{
        boolean_t needs_update = FALSE;
        thread_t thread = *in_thread;
        struct turnstile *waiting_turnstile = TURNSTILE_NULL;
        uint32_t turnstile_gencount;
        boolean_t first_update = !total_hop;

        *in_thread = THREAD_NULL;

        /* Check if thread waiting on a turnstile */
        waiting_turnstile = thread_get_waiting_turnstile(thread);

        if (waiting_turnstile == TURNSTILE_NULL || thread_hop > turnstile_max_hop) {
                KERNEL_DEBUG_CONSTANT_IST(KDEBUG_TRACE,
                    (TURNSTILE_CODE(TURNSTILE_HEAP_OPERATIONS,
                    (waiting_turnstile ? TURNSTILE_UPDATE_STOPPED_BY_LIMIT : THREAD_NOT_WAITING_ON_TURNSTILE)
                    )) | DBG_FUNC_NONE,
                    thread_tid(thread),
                    turnstile_max_hop,
                    thread_hop,
                    VM_KERNEL_UNSLIDE_OR_PERM(waiting_turnstile), 0);
                turnstile_stats_update(total_hop + 1, TSU_NO_TURNSTILE |
                    TSU_THREAD_ARG | tsu_flags, thread);
                thread_unlock(thread);
                return;
        }

        /* Check if the thread needs to update the waiting turnstile */
        needs_update = turnstile_need_thread_promotion_update(waiting_turnstile, thread);
        if (!needs_update && !first_update) {
                turnstile_stats_update(total_hop + 1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_THREAD_ARG | tsu_flags, thread);
                thread_unlock(thread);
                return;
        }

        /* take a reference on thread, turnstile and snapshot of gencount */
        turnstile_gencount = turnstile_get_gencount(waiting_turnstile);
        turnstile_reference(waiting_turnstile);
        thread_reference(thread);

        /* drop the thread lock and acquire the turnstile lock */
        thread_unlock(thread);
        waitq_lock(&waiting_turnstile->ts_waitq);
        thread_lock(thread);

        /* Check if the gencount matches and thread is still waiting on same turnstile */
        if (turnstile_gencount != turnstile_get_gencount(waiting_turnstile) ||
            waiting_turnstile != thread_get_waiting_turnstile(thread)) {
                turnstile_stats_update(total_hop + 1, TSU_NO_PRI_CHANGE_NEEDED |
                    TSU_THREAD_ARG | tsu_flags, thread);
                /* No updates required, bail out */
                thread_unlock(thread);
                waitq_unlock(&waiting_turnstile->ts_waitq);
                thread_deallocate_safe(thread);
                turnstile_deallocate_safe(waiting_turnstile);
                return;
        }

        /*
         * The thread is waiting on the waiting_turnstile and we have thread lock,
         * we can drop the thread and turnstile reference since its on waitq and
         * it could not be removed from the waitq without the thread lock.
         */
        thread_deallocate_safe(thread);
        turnstile_deallocate_safe(waiting_turnstile);

        /* adjust thread's position on turnstile waitq */
        needs_update = turnstile_update_thread_promotion_locked(waiting_turnstile, thread);

        /*
         * Check if thread needs further priority propagation,
         * since the first hop priority update was done in
         * turnstile_update_inheritor, do not bailout if it is
         * the first update as needs_update flag would evaluate to
         * false for that case.
         */
        if (!needs_update && !first_update) {
                turnstile_stats_update(total_hop + 1,
                    (waiting_turnstile->ts_inheritor ? TSU_NO_PRI_CHANGE_NEEDED : TSU_NO_INHERITOR) |
                    TSU_THREAD_ARG | tsu_flags, thread);
                thread_unlock(thread);
                waitq_unlock(&waiting_turnstile->ts_waitq);
                return;
        }

        /* drop the thread lock and update the turnstile */
        thread_unlock(thread);
        *out_turnstile = waiting_turnstile;
}

/*
 * Name: turnstile_stats_update
 *
 * Description: Function to update turnstile stats for dev kernel.
 *
 * Arg1: hops : number of thread hops in priority propagation
 * Arg2: flags : turnstile stats update flags
 * Arg3: inheritor: inheritor
 *
 * Returns: Nothing
 */
void
turnstile_stats_update(
        int hop __assert_only,
        turnstile_stats_update_flags_t flags __assert_only,
        turnstile_inheritor_t inheritor __assert_only)
{
#if DEVELOPMENT || DEBUG
        if (flags & TSU_TURNSTILE_BLOCK_COUNT) {
                os_atomic_inc(&thread_block_on_turnstile_count, relaxed);
        }

        if (flags & TSU_REGULAR_WAITQ_BLOCK_COUNT) {
                os_atomic_inc(&thread_block_on_regular_waitq_count, relaxed);
        }

        if (hop > TURNSTILE_MAX_HOP_DEFAULT || hop == 0) {
                return;
        }

        assert(hop >= 0);

        /*
         * Check if turnstile stats needs to be updated.
         * Bail out if the turnstile or thread does not
         * have any user promotion.
         * Bail out if it is the first hop of WQ turnstile
         * since WQ's use of a turnstile for the admission check
         * introduces a lot of noise due to state changes.
         */
        if (flags & TSU_TURNSTILE_ARG) {
                struct turnstile *ts = (struct turnstile *)inheritor;
                if (ts->ts_priority == 0) {
                        return;
                }

                if (hop == 1 && turnstile_get_type(ts) == TURNSTILE_WORKQS) {
                        return;
                }
        } else if (flags & TSU_THREAD_ARG) {
                thread_t thread = (thread_t)inheritor;
                if (thread->user_promotion_basepri == 0) {
                        return;
                }
        } else {
                assert(inheritor == NULL);
        }

        struct turnstile_stats *turnstile_stats;
        if (flags & TSU_BOOST_ARG) {
                turnstile_stats = turnstile_boost_stats;
        } else {
                turnstile_stats = turnstile_unboost_stats;
        }

        if (flags & TSU_PRI_PROPAGATION) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_priority_propagation, relaxed);
        }

        if (flags & TSU_NO_INHERITOR) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_no_inheritor, relaxed);
        }

        if (flags & TSU_NO_TURNSTILE) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_no_turnstile, relaxed);
        }

        if (flags & TSU_NO_PRI_CHANGE_NEEDED) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_no_priority_change_required, relaxed);
        }

        if (flags & TSU_THREAD_RUNNABLE) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_thread_runnable, relaxed);
        }

        if (flags & TSU_ABOVE_UI_PRI_CHANGE) {
                os_atomic_inc(&turnstile_stats[hop - 1].ts_above_ui_pri_change, relaxed);
        }
#endif
}

static uint64_t
kdp_turnstile_traverse_inheritor_chain(struct turnstile *ts, uint64_t *flags, uint8_t *hops)
{
        if (waitq_held(&ts->ts_waitq)) {
                *flags |= STACKSHOT_TURNSTILE_STATUS_LOCKED_WAITQ;
                return 0;
        }

        *hops = *hops + 1;

        if (ts->ts_inheritor_flags & TURNSTILE_INHERITOR_TURNSTILE) {
                return kdp_turnstile_traverse_inheritor_chain(ts->ts_inheritor, flags, hops);
        }

        if (ts->ts_inheritor_flags & TURNSTILE_INHERITOR_THREAD) {
                *flags |= STACKSHOT_TURNSTILE_STATUS_THREAD;
                return (uint64_t) thread_tid(ts->ts_inheritor);
        }

        if (ts->ts_inheritor_flags & TURNSTILE_INHERITOR_WORKQ) {
                *flags |= STACKSHOT_TURNSTILE_STATUS_WORKQUEUE;
                return VM_KERNEL_UNSLIDE_OR_PERM(ts->ts_inheritor);
        }

        *flags |= STACKSHOT_TURNSTILE_STATUS_UNKNOWN;
        return 0;
}

void
kdp_turnstile_fill_tsinfo(struct turnstile *ts, thread_turnstileinfo_t *tsinfo)
{
        uint64_t final_inheritor;
        uint64_t flags = 0;
        uint8_t hops = 0;

        tsinfo->turnstile_context  = 0;
        tsinfo->number_of_hops     = 0;
        tsinfo->turnstile_priority = 0;

        assert(ts != TURNSTILE_NULL);

        if (waitq_held(&ts->ts_waitq)) {
                tsinfo->turnstile_flags |= STACKSHOT_TURNSTILE_STATUS_LOCKED_WAITQ;
                return;
        }

        final_inheritor = kdp_turnstile_traverse_inheritor_chain(ts, &flags, &hops);

        /* store some metadata about the turnstile itself */
        tsinfo->turnstile_flags = flags;
        tsinfo->number_of_hops = hops;
        tsinfo->turnstile_priority = ts->ts_priority;
        tsinfo->turnstile_context = final_inheritor;
}

#if DEVELOPMENT || DEBUG

int sysctl_io_opaque(void *req, void *pValue, size_t valueSize, int *changed);

/*
 * Name: turnstile_get_boost_stats_sysctl
 *
 * Description: Function to get turnstile stats.
 *
 * Args: req : opaque struct to pass to sysctl_io_opaque
 *
 * Returns: errorno
 */
int
turnstile_get_boost_stats_sysctl(
        void *req)
{
        return sysctl_io_opaque(req, turnstile_boost_stats, sizeof(struct turnstile_stats) * TURNSTILE_MAX_HOP_DEFAULT, NULL);
}

/*
 * Name: get_turnstile_stats_sysctl
 *
 * Description: Function to get turnstile stats.
 *
 * Args: req : opaque struct to pass to sysctl_io_opaque
 *
 * Returns: errorno
 */
int
turnstile_get_unboost_stats_sysctl(
        void *req)
{
        return sysctl_io_opaque(req, turnstile_unboost_stats, sizeof(struct turnstile_stats) * TURNSTILE_MAX_HOP_DEFAULT, NULL);
}

/* Testing interface for Development kernels */
#define tstile_test_prim_lock_interlock(test_prim) \
        lck_spin_lock(&test_prim->ttprim_interlock)
#define tstile_test_prim_unlock_interlock(test_prim) \
        lck_spin_unlock(&test_prim->ttprim_interlock)

static void
tstile_test_prim_init(struct tstile_test_prim **test_prim_ptr)
{
        struct tstile_test_prim *test_prim = (struct tstile_test_prim *) kalloc(sizeof(struct tstile_test_prim));

        test_prim->ttprim_turnstile = TURNSTILE_NULL;
        test_prim->ttprim_owner = NULL;
        lck_spin_init(&test_prim->ttprim_interlock, &turnstiles_dev_lock_grp, &turnstiles_dev_lock_attr);
        test_prim->tt_prim_waiters = 0;

        *test_prim_ptr = test_prim;
        return;
}

int
tstile_test_prim_lock(int val)
{
        struct tstile_test_prim *test_prim;
        boolean_t use_hashtable;
        turnstile_type_t type;
        wait_interrupt_t wait_type;

        switch (val) {
        case SYSCTL_TURNSTILE_TEST_USER_DEFAULT:
                test_prim = test_prim_ts_inline;
                use_hashtable = FALSE;
                wait_type = THREAD_ABORTSAFE;
                type = TURNSTILE_ULOCK;
                break;
        case SYSCTL_TURNSTILE_TEST_USER_HASHTABLE:
                test_prim = test_prim_global_htable;
                use_hashtable = TRUE;
                wait_type = THREAD_ABORTSAFE;
                type = TURNSTILE_ULOCK;
                break;
        case SYSCTL_TURNSTILE_TEST_KERNEL_DEFAULT:
                test_prim = test_prim_global_ts_kernel;
                use_hashtable = FALSE;
                wait_type = THREAD_UNINT | THREAD_WAIT_NOREPORT_USER;
                type = TURNSTILE_KERNEL_MUTEX;
                break;
        case SYSCTL_TURNSTILE_TEST_KERNEL_HASHTABLE:
                test_prim = test_prim_global_ts_kernel_hash;
                use_hashtable = TRUE;
                wait_type = THREAD_UNINT | THREAD_WAIT_NOREPORT_USER;
                type = TURNSTILE_KERNEL_MUTEX;
                break;

        default:
                return -1;
        }

lock_start:

        /* take the interlock of the primitive */
        tstile_test_prim_lock_interlock(test_prim);

        /* Check if the lock is available */
        if (test_prim->ttprim_owner == NULL && test_prim->tt_prim_waiters == 0) {
                thread_reference(current_thread());
                test_prim->ttprim_owner = current_thread();
                tstile_test_prim_unlock_interlock(test_prim);
                return 0;
        }

        struct turnstile *prim_turnstile = TURNSTILE_NULL;

        /* primitive locked, get a turnstile */
        prim_turnstile = turnstile_prepare((uintptr_t)test_prim,
            use_hashtable ? NULL : &test_prim->ttprim_turnstile,
            TURNSTILE_NULL, type);

        assert(prim_turnstile != TURNSTILE_NULL);

        /* This is contented acquire case */
        if (test_prim->ttprim_owner == NULL) {
                thread_reference(current_thread());
                test_prim->ttprim_owner = current_thread();

                /* Update the turnstile owner */
                turnstile_update_inheritor(prim_turnstile,
                    current_thread(),
                    (TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_THREAD));

                turnstile_update_inheritor_complete(prim_turnstile, TURNSTILE_INTERLOCK_HELD);

                turnstile_complete((uintptr_t)test_prim,
                    use_hashtable ? NULL : &test_prim->ttprim_turnstile, NULL, type);

                tstile_test_prim_unlock_interlock(test_prim);

                turnstile_cleanup();
                return 0;
        }

        test_prim->tt_prim_waiters++;
        turnstile_update_inheritor(prim_turnstile,
            test_prim->ttprim_owner,
            (TURNSTILE_DELAYED_UPDATE | TURNSTILE_INHERITOR_THREAD));

        waitq_assert_wait64(&prim_turnstile->ts_waitq,
            CAST_EVENT64_T(test_prim), wait_type,
            TIMEOUT_WAIT_FOREVER);

        /* drop the interlock */
        tstile_test_prim_unlock_interlock(test_prim);

        turnstile_update_inheritor_complete(prim_turnstile, TURNSTILE_INTERLOCK_NOT_HELD);

        wait_result_t result;
        result = thread_block(THREAD_CONTINUE_NULL);

        /* re-acquire the interlock to get turnstile back */
        tstile_test_prim_lock_interlock(test_prim);
        test_prim->tt_prim_waiters--;
        turnstile_complete((uintptr_t)test_prim,
            use_hashtable ? NULL : &test_prim->ttprim_turnstile, NULL, type);

        tstile_test_prim_unlock_interlock(test_prim);

        turnstile_cleanup();

        /* Return if thread interrupted */
        if (result == THREAD_INTERRUPTED) {
                return 1;
        }

        goto lock_start;
}

int
tstile_test_prim_unlock(int val)
{
        struct tstile_test_prim *test_prim;
        boolean_t use_hashtable;
        turnstile_type_t type;

        switch (val) {
        case SYSCTL_TURNSTILE_TEST_USER_DEFAULT:
                test_prim = test_prim_ts_inline;
                use_hashtable = FALSE;
                type = TURNSTILE_ULOCK;
                break;
        case SYSCTL_TURNSTILE_TEST_USER_HASHTABLE:
                test_prim = test_prim_global_htable;
                use_hashtable = TRUE;
                type = TURNSTILE_ULOCK;
                break;
        case SYSCTL_TURNSTILE_TEST_KERNEL_DEFAULT:
                test_prim = test_prim_global_ts_kernel;
                use_hashtable = FALSE;
                type = TURNSTILE_KERNEL_MUTEX;
                break;
        case SYSCTL_TURNSTILE_TEST_KERNEL_HASHTABLE:
                test_prim = test_prim_global_ts_kernel_hash;
                use_hashtable = TRUE;
                type = TURNSTILE_KERNEL_MUTEX;
                break;
        default:
                return -1;
        }

        /* take the interlock of the primitive */
        tstile_test_prim_lock_interlock(test_prim);

        if (test_prim->ttprim_owner == NULL) {
                tstile_test_prim_unlock_interlock(test_prim);
                return 1;
        }

        /* Check if the lock is contended */
        if (test_prim->ttprim_owner != NULL && test_prim->tt_prim_waiters == 0) {
                /* lock is not contended */
                thread_t old_owner = test_prim->ttprim_owner;
                test_prim->ttprim_owner = NULL;
                tstile_test_prim_unlock_interlock(test_prim);

                thread_deallocate(old_owner);
                return 0;
        }

        struct turnstile *prim_turnstile = TURNSTILE_NULL;

        thread_t old_owner = test_prim->ttprim_owner;
        test_prim->ttprim_owner = NULL;

        /* primitive locked, get a turnstile */
        prim_turnstile = turnstile_prepare((uintptr_t)test_prim,
            use_hashtable ? NULL : &test_prim->ttprim_turnstile,
            TURNSTILE_NULL, type);

        assert(prim_turnstile != TURNSTILE_NULL);

        /* Update the turnstile owner */
        turnstile_update_inheritor(prim_turnstile,
            NULL,
            (TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_THREAD));

        waitq_wakeup64_one(&prim_turnstile->ts_waitq,
            CAST_EVENT64_T(test_prim),
            THREAD_AWAKENED, WAITQ_ALL_PRIORITIES);

        turnstile_update_inheritor_complete(prim_turnstile, TURNSTILE_INTERLOCK_HELD);

        turnstile_complete((uintptr_t)test_prim,
            use_hashtable ? NULL : &test_prim->ttprim_turnstile, NULL, type);

        tstile_test_prim_unlock_interlock(test_prim);

        turnstile_cleanup();

        if (old_owner) {
                /* Changing this to thread_deallocate_safe to exercise thread_deallocate_safe path */
                thread_deallocate_safe(old_owner);
        }

        return 0;
}

#endif