libdispatch-84.5.tar.gz

[apple/libdispatch.git] / src / queue.c

/*
 * Copyright (c) 2008-2009 Apple Inc. All rights reserved.
 *
 * @APPLE_APACHE_LICENSE_HEADER_START@
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 * @APPLE_APACHE_LICENSE_HEADER_END@
 */

#include "internal.h"
#include "protocol.h"

void
dummy_function(void)
{
}

long
dummy_function_r0(void)
{
        return 0;
}

static bool _dispatch_select_workaround;
static fd_set _dispatch_rfds;
static fd_set _dispatch_wfds;
static void *_dispatch_rfd_ptrs[FD_SETSIZE];
static void *_dispatch_wfd_ptrs[FD_SETSIZE];


static struct dispatch_semaphore_s _dispatch_thread_mediator[] = {
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
        {
                .do_vtable = &_dispatch_semaphore_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        },
};

static struct dispatch_queue_s _dispatch_root_queues[];

static inline dispatch_queue_t
_dispatch_get_root_queue(long priority, bool overcommit)
{
        if (overcommit) switch (priority) {
        case DISPATCH_QUEUE_PRIORITY_LOW:
                return &_dispatch_root_queues[1];
        case DISPATCH_QUEUE_PRIORITY_DEFAULT:
                return &_dispatch_root_queues[3];
        case DISPATCH_QUEUE_PRIORITY_HIGH:
                return &_dispatch_root_queues[5];
        }
        switch (priority) {
        case DISPATCH_QUEUE_PRIORITY_LOW:
                return &_dispatch_root_queues[0];
        case DISPATCH_QUEUE_PRIORITY_DEFAULT:
                return &_dispatch_root_queues[2];
        case DISPATCH_QUEUE_PRIORITY_HIGH:
                return &_dispatch_root_queues[4];
        default:
                return NULL;
        }
}

#ifdef __BLOCKS__
dispatch_block_t
_dispatch_Block_copy(dispatch_block_t db)
{
        dispatch_block_t rval;

        while (!(rval = Block_copy(db))) {
                sleep(1);
        }

        return rval;
}
#define _dispatch_Block_copy(x) ((typeof(x))_dispatch_Block_copy(x))

void
_dispatch_call_block_and_release(void *block)
{
        void (^b)(void) = block;
        b();
        Block_release(b);
}

void
_dispatch_call_block_and_release2(void *block, void *ctxt)
{
        void (^b)(void*) = block;
        b(ctxt);
        Block_release(b);
}

#endif /* __BLOCKS__ */

struct dispatch_queue_attr_vtable_s {
        DISPATCH_VTABLE_HEADER(dispatch_queue_attr_s);
};

struct dispatch_queue_attr_s {
        DISPATCH_STRUCT_HEADER(dispatch_queue_attr_s, dispatch_queue_attr_vtable_s);

        // Public:
        int qa_priority;
        void* finalizer_ctxt;
        dispatch_queue_finalizer_function_t finalizer_func;

        // Private:
        unsigned long qa_flags;
};

static int _dispatch_pthread_sigmask(int how, sigset_t *set, sigset_t *oset);

#define _dispatch_queue_trylock(dq) dispatch_atomic_cmpxchg(&(dq)->dq_running, 0, 1)
static inline void _dispatch_queue_unlock(dispatch_queue_t dq);
static void _dispatch_queue_invoke(dispatch_queue_t dq);
static void _dispatch_queue_serial_drain_till_empty(dispatch_queue_t dq);
static bool _dispatch_queue_wakeup_global(dispatch_queue_t dq);
static struct dispatch_object_s *_dispatch_queue_concurrent_drain_one(dispatch_queue_t dq);

static bool _dispatch_program_is_probably_callback_driven;

#if DISPATCH_COCOA_COMPAT
void (*dispatch_begin_thread_4GC)(void) = dummy_function;
void (*dispatch_end_thread_4GC)(void) = dummy_function;
void *(*_dispatch_begin_NSAutoReleasePool)(void) = (void *)dummy_function;
void (*_dispatch_end_NSAutoReleasePool)(void *) = (void *)dummy_function;
static void _dispatch_queue_wakeup_main(void);

static dispatch_once_t _dispatch_main_q_port_pred;
static bool main_q_is_draining;
static mach_port_t main_q_port;
#endif

static void _dispatch_cache_cleanup2(void *value);
static void _dispatch_force_cache_cleanup(void);

static const struct dispatch_queue_vtable_s _dispatch_queue_vtable = {
        .do_type = DISPATCH_QUEUE_TYPE,
        .do_kind = "queue",
        .do_dispose = _dispatch_queue_dispose,
        .do_invoke = (void *)dummy_function_r0,
        .do_probe = (void *)dummy_function_r0,
        .do_debug = dispatch_queue_debug,
};

static const struct dispatch_queue_vtable_s _dispatch_queue_root_vtable = {
        .do_type = DISPATCH_QUEUE_GLOBAL_TYPE,
        .do_kind = "global-queue",
        .do_debug = dispatch_queue_debug,
        .do_probe = _dispatch_queue_wakeup_global,
};

#define MAX_THREAD_COUNT 255

struct dispatch_root_queue_context_s {
        pthread_workqueue_t dgq_kworkqueue;
        uint32_t dgq_pending;
        uint32_t dgq_thread_pool_size;
        dispatch_semaphore_t dgq_thread_mediator;
};

#define DISPATCH_ROOT_QUEUE_COUNT (DISPATCH_QUEUE_PRIORITY_COUNT * 2)
static struct dispatch_root_queue_context_s _dispatch_root_queue_contexts[] = {
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[0],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[1],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[2],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[3],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[4],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
        {
                .dgq_thread_mediator = &_dispatch_thread_mediator[5],
                .dgq_thread_pool_size = MAX_THREAD_COUNT,
        },
};

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
// dq_running is set to 2 so that barrier operations go through the slow path
static struct dispatch_queue_s _dispatch_root_queues[] = {
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[0],

                .dq_label = "com.apple.root.low-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 4,
        },
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[1],

                .dq_label = "com.apple.root.low-overcommit-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 5,
        },
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[2],

                .dq_label = "com.apple.root.default-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 6,
        },
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[3],

                .dq_label = "com.apple.root.default-overcommit-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 7,
        },
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[4],

                .dq_label = "com.apple.root.high-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 8,
        },
        {
                .do_vtable = &_dispatch_queue_root_vtable,
                .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
                .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
                .do_ctxt = &_dispatch_root_queue_contexts[5],

                .dq_label = "com.apple.root.high-overcommit-priority",
                .dq_running = 2,
                .dq_width = UINT32_MAX,
                .dq_serialnum = 9,
        },
};

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
struct dispatch_queue_s _dispatch_main_q = {
        .do_vtable = &_dispatch_queue_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_targetq = &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_COUNT / 2],

        .dq_label = "com.apple.main-thread",
        .dq_running = 1,
        .dq_width = 1,
        .dq_serialnum = 1,
};

#if DISPATCH_PERF_MON
static OSSpinLock _dispatch_stats_lock;
static size_t _dispatch_bad_ratio;
static struct {
        uint64_t time_total;
        uint64_t count_total;
        uint64_t thread_total;
} _dispatch_stats[65]; // ffs*/fls*() returns zero when no bits are set
static void _dispatch_queue_merge_stats(uint64_t start);
#endif

static void *_dispatch_worker_thread(void *context);
static void _dispatch_worker_thread2(void *context);

malloc_zone_t *_dispatch_ccache_zone;

static inline void
_dispatch_continuation_free(dispatch_continuation_t dc)
{
        dispatch_continuation_t prev_dc = _dispatch_thread_getspecific(dispatch_cache_key);
        dc->do_next = prev_dc;
        _dispatch_thread_setspecific(dispatch_cache_key, dc);
}

static inline void
_dispatch_continuation_pop(dispatch_object_t dou)
{
        dispatch_continuation_t dc = dou._dc;
        dispatch_group_t dg;

        if (DISPATCH_OBJ_IS_VTABLE(dou._do)) {
                return _dispatch_queue_invoke(dou._dq);
        }

        // Add the item back to the cache before calling the function. This
        // allows the 'hot' continuation to be used for a quick callback.
        //
        // The ccache version is per-thread.
        // Therefore, the object has not been reused yet.
        // This generates better assembly.
        if ((long)dou._do->do_vtable & DISPATCH_OBJ_ASYNC_BIT) {
                _dispatch_continuation_free(dc);
        }
        if ((long)dou._do->do_vtable & DISPATCH_OBJ_GROUP_BIT) {
                dg = dc->dc_group;
        } else {
                dg = NULL;
        }
        dc->dc_func(dc->dc_ctxt);
        if (dg) {
                dispatch_group_leave(dg);
                _dispatch_release(dg);
        }
}

struct dispatch_object_s *
_dispatch_queue_concurrent_drain_one(dispatch_queue_t dq)
{
        struct dispatch_object_s *head, *next, *const mediator = (void *)~0ul;

        // The mediator value acts both as a "lock" and a signal
        head = dispatch_atomic_xchg(&dq->dq_items_head, mediator);

        if (slowpath(head == NULL)) {
                // The first xchg on the tail will tell the enqueueing thread that it
                // is safe to blindly write out to the head pointer. A cmpxchg honors
                // the algorithm.
                dispatch_atomic_cmpxchg(&dq->dq_items_head, mediator, NULL);
                _dispatch_debug("no work on global work queue");
                return NULL;
        }

        if (slowpath(head == mediator)) {
                // This thread lost the race for ownership of the queue.
                //
                // The ratio of work to libdispatch overhead must be bad. This
                // scenario implies that there are too many threads in the pool.
                // Create a new pending thread and then exit this thread.
                // The kernel will grant a new thread when the load subsides.
                _dispatch_debug("Contention on queue: %p", dq);
                _dispatch_queue_wakeup_global(dq);
#if DISPATCH_PERF_MON
                dispatch_atomic_inc(&_dispatch_bad_ratio);
#endif
                return NULL;
        }

        // Restore the head pointer to a sane value before returning.
        // If 'next' is NULL, then this item _might_ be the last item.
        next = fastpath(head->do_next);

        if (slowpath(!next)) {
                dq->dq_items_head = NULL;
                
                if (dispatch_atomic_cmpxchg(&dq->dq_items_tail, head, NULL)) {
                        // both head and tail are NULL now
                        goto out;
                }

                // There must be a next item now. This thread won't wait long.
                while (!(next = head->do_next)) {
                        _dispatch_hardware_pause();
                }
        }

        dq->dq_items_head = next;
        _dispatch_queue_wakeup_global(dq);
out:
        return head;
}

dispatch_queue_t
dispatch_get_current_queue(void)
{
        return _dispatch_queue_get_current() ?: _dispatch_get_root_queue(0, true);
}

#undef dispatch_get_main_queue
__OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_NA)
dispatch_queue_t dispatch_get_main_queue(void);

dispatch_queue_t
dispatch_get_main_queue(void)
{
        return &_dispatch_main_q;
}
#define dispatch_get_main_queue() (&_dispatch_main_q)

struct _dispatch_hw_config_s _dispatch_hw_config;

static void
_dispatch_queue_set_width_init(void)
{
        size_t valsz = sizeof(uint32_t);

        errno = 0;
        sysctlbyname("hw.activecpu", &_dispatch_hw_config.cc_max_active, &valsz, NULL, 0);
        dispatch_assume_zero(errno);
        dispatch_assume(valsz == sizeof(uint32_t));

        errno = 0;
        sysctlbyname("hw.logicalcpu_max", &_dispatch_hw_config.cc_max_logical, &valsz, NULL, 0);
        dispatch_assume_zero(errno);
        dispatch_assume(valsz == sizeof(uint32_t));

        errno = 0;
        sysctlbyname("hw.physicalcpu_max", &_dispatch_hw_config.cc_max_physical, &valsz, NULL, 0);
        dispatch_assume_zero(errno);
        dispatch_assume(valsz == sizeof(uint32_t));
}

void
dispatch_queue_set_width(dispatch_queue_t dq, long width)
{
        int w = (int)width;     // intentional truncation
        uint32_t tmp;

        if (slowpath(dq->do_ref_cnt == DISPATCH_OBJECT_GLOBAL_REFCNT)) {
                return;
        }
        if (w == 1 || w == 0) {
                dq->dq_width = 1;
                return;
        }
        if (w > 0) {
                tmp = w;
        } else switch (w) {
        case DISPATCH_QUEUE_WIDTH_MAX_PHYSICAL_CPUS:
                tmp = _dispatch_hw_config.cc_max_physical;
                break;
        case DISPATCH_QUEUE_WIDTH_ACTIVE_CPUS:
                tmp = _dispatch_hw_config.cc_max_active;
                break;
        default:
                // fall through
        case DISPATCH_QUEUE_WIDTH_MAX_LOGICAL_CPUS:
                tmp = _dispatch_hw_config.cc_max_logical;
                break;
        }
        // multiply by two since the running count is inc/dec by two (the low bit == barrier)
        dq->dq_width = tmp * 2;

        // XXX if the queue has items and the width is increased, we should try to wake the queue
}

// skip zero
// 1 - main_q
// 2 - mgr_q
// 3 - _unused_
// 4,5,6,7,8,9 - global queues
// we use 'xadd' on Intel, so the initial value == next assigned
static unsigned long _dispatch_queue_serial_numbers = 10;

// Note to later developers: ensure that any initialization changes are
// made for statically allocated queues (i.e. _dispatch_main_q).
inline void
_dispatch_queue_init(dispatch_queue_t dq)
{
        dq->do_vtable = &_dispatch_queue_vtable;
        dq->do_next = DISPATCH_OBJECT_LISTLESS;
        dq->do_ref_cnt = 1;
        dq->do_xref_cnt = 1;
        dq->do_targetq = _dispatch_get_root_queue(0, true);
        dq->dq_running = 0;
        dq->dq_width = 1;
        dq->dq_serialnum = dispatch_atomic_inc(&_dispatch_queue_serial_numbers) - 1;
}

dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
        dispatch_queue_t dq;
        size_t label_len;

        if (!label) {
                label = "";
        }

        label_len = strlen(label);
        if (label_len < (DISPATCH_QUEUE_MIN_LABEL_SIZE - 1)) {
                label_len = (DISPATCH_QUEUE_MIN_LABEL_SIZE - 1);
        }

        // XXX switch to malloc()
        dq = calloc(1ul, sizeof(struct dispatch_queue_s) - DISPATCH_QUEUE_MIN_LABEL_SIZE + label_len + 1);
        if (slowpath(!dq)) {
                return dq;
        }

        _dispatch_queue_init(dq);
        strcpy(dq->dq_label, label);

#ifndef DISPATCH_NO_LEGACY
        if (slowpath(attr)) {
                dq->do_targetq = _dispatch_get_root_queue(attr->qa_priority, attr->qa_flags & DISPATCH_QUEUE_OVERCOMMIT);
                dq->dq_finalizer_ctxt = attr->finalizer_ctxt;
                dq->dq_finalizer_func = attr->finalizer_func;
#ifdef __BLOCKS__
                if (attr->finalizer_func == (void*)_dispatch_call_block_and_release2) {
                        // if finalizer_ctxt is a Block, retain it.
                        dq->dq_finalizer_ctxt = Block_copy(dq->dq_finalizer_ctxt);
                        if (!(dq->dq_finalizer_ctxt)) {
                                goto out_bad;
                        }
                }
#endif
        }
#endif

        return dq;

out_bad:
        free(dq);
        return NULL;
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
void
_dispatch_queue_dispose(dispatch_queue_t dq)
{
        if (slowpath(dq == _dispatch_queue_get_current())) {
                DISPATCH_CRASH("Release of a queue by itself");
        }
        if (slowpath(dq->dq_items_tail)) {
                DISPATCH_CRASH("Release of a queue while items are enqueued");
        }

#ifndef DISPATCH_NO_LEGACY
        if (dq->dq_finalizer_func) {
                dq->dq_finalizer_func(dq->dq_finalizer_ctxt, dq);
        }
#endif

        // trash the tail queue so that use after free will crash
        dq->dq_items_tail = (void *)0x200;

        _dispatch_dispose(dq);
}

DISPATCH_NOINLINE
static void
_dispatch_barrier_async_f_slow(dispatch_queue_t dq, void *context, dispatch_function_t func)
{
        dispatch_continuation_t dc = fastpath(_dispatch_continuation_alloc_from_heap());

        dc->do_vtable = (void *)(DISPATCH_OBJ_ASYNC_BIT | DISPATCH_OBJ_BARRIER_BIT);
        dc->dc_func = func;
        dc->dc_ctxt = context;

        _dispatch_queue_push(dq, dc);
}

#ifdef __BLOCKS__
void
dispatch_barrier_async(dispatch_queue_t dq, void (^work)(void))
{
        dispatch_barrier_async_f(dq, _dispatch_Block_copy(work), _dispatch_call_block_and_release);
}
#endif

DISPATCH_NOINLINE
void
dispatch_barrier_async_f(dispatch_queue_t dq, void *context, dispatch_function_t func)
{
        dispatch_continuation_t dc = fastpath(_dispatch_continuation_alloc_cacheonly());

        if (!dc) {
                return _dispatch_barrier_async_f_slow(dq, context, func);
        }

        dc->do_vtable = (void *)(DISPATCH_OBJ_ASYNC_BIT | DISPATCH_OBJ_BARRIER_BIT);
        dc->dc_func = func;
        dc->dc_ctxt = context;

        _dispatch_queue_push(dq, dc);
}

DISPATCH_NOINLINE
static void
_dispatch_async_f_slow(dispatch_queue_t dq, void *context, dispatch_function_t func)
{
        dispatch_continuation_t dc = fastpath(_dispatch_continuation_alloc_from_heap());

        dc->do_vtable = (void *)DISPATCH_OBJ_ASYNC_BIT;
        dc->dc_func = func;
        dc->dc_ctxt = context;

        _dispatch_queue_push(dq, dc);
}

#ifdef __BLOCKS__
void
dispatch_async(dispatch_queue_t dq, void (^work)(void))
{
        dispatch_async_f(dq, _dispatch_Block_copy(work), _dispatch_call_block_and_release);
}
#endif

DISPATCH_NOINLINE
void
dispatch_async_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func)
{
        dispatch_continuation_t dc = fastpath(_dispatch_continuation_alloc_cacheonly());

        // unlike dispatch_sync_f(), we do NOT need to check the queue width,
        // the "drain" function will do this test

        if (!dc) {
                return _dispatch_async_f_slow(dq, ctxt, func);
        }

        dc->do_vtable = (void *)DISPATCH_OBJ_ASYNC_BIT;
        dc->dc_func = func;
        dc->dc_ctxt = ctxt;

        _dispatch_queue_push(dq, dc);
}

struct dispatch_barrier_sync_slow2_s {
        dispatch_function_t dbss2_func;
        dispatch_function_t dbss2_ctxt;
        dispatch_semaphore_t dbss2_sema;
};

static void
_dispatch_barrier_sync_f_slow_invoke(void *ctxt)
{
        struct dispatch_barrier_sync_slow2_s *dbss2 = ctxt;

        dbss2->dbss2_func(dbss2->dbss2_ctxt);
        dispatch_semaphore_signal(dbss2->dbss2_sema);
}

DISPATCH_NOINLINE
static void
_dispatch_barrier_sync_f_slow(dispatch_queue_t dq, void *ctxt, dispatch_function_t func)
{
        struct dispatch_barrier_sync_slow2_s dbss2 = {
                .dbss2_func = func,
                .dbss2_ctxt = ctxt,
                .dbss2_sema = _dispatch_get_thread_semaphore(),
        };
        struct dispatch_barrier_sync_slow_s {
                DISPATCH_CONTINUATION_HEADER(dispatch_barrier_sync_slow_s);
        } dbss = {
                .do_vtable = (void *)DISPATCH_OBJ_BARRIER_BIT,
                .dc_func = _dispatch_barrier_sync_f_slow_invoke,
                .dc_ctxt = &dbss2,
        };

        _dispatch_queue_push(dq, (void *)&dbss);

        while (dispatch_semaphore_wait(dbss2.dbss2_sema, dispatch_time(0, 3ull * NSEC_PER_SEC))) {
                if (DISPATCH_OBJECT_SUSPENDED(dq)) {
                        continue;
                }
                if (_dispatch_queue_trylock(dq)) {
                        _dispatch_queue_drain(dq);
                        _dispatch_queue_unlock(dq);
                }
        }
        _dispatch_put_thread_semaphore(dbss2.dbss2_sema);
}

#ifdef __BLOCKS__
void
dispatch_barrier_sync(dispatch_queue_t dq, void (^work)(void))
{
        struct Block_basic *bb = (void *)work;

        dispatch_barrier_sync_f(dq, work, (dispatch_function_t)bb->Block_invoke);
}
#endif

DISPATCH_NOINLINE
void
dispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func)
{
        dispatch_queue_t old_dq = _dispatch_thread_getspecific(dispatch_queue_key);

        // 1) ensure that this thread hasn't enqueued anything ahead of this call
        // 2) the queue is not suspended
        // 3) the queue is not weird
        if (slowpath(dq->dq_items_tail)
                        || slowpath(DISPATCH_OBJECT_SUSPENDED(dq))
                        || slowpath(!_dispatch_queue_trylock(dq))) {
                return _dispatch_barrier_sync_f_slow(dq, ctxt, func);
        }

        _dispatch_thread_setspecific(dispatch_queue_key, dq);
        func(ctxt);
        _dispatch_workitem_inc();
        _dispatch_thread_setspecific(dispatch_queue_key, old_dq);
        _dispatch_queue_unlock(dq);
}

static void
_dispatch_sync_f_slow2(void *ctxt)
{
        dispatch_queue_t dq = _dispatch_queue_get_current();
        dispatch_atomic_add(&dq->dq_running, 2);
        dispatch_semaphore_signal(ctxt);
}

DISPATCH_NOINLINE
static void
_dispatch_sync_f_slow(dispatch_queue_t dq)
{
        // the global root queues do not need strict ordering
        if (dq->do_targetq == NULL) {
                dispatch_atomic_add(&dq->dq_running, 2);
                return;
        }

        struct dispatch_sync_slow_s {
                DISPATCH_CONTINUATION_HEADER(dispatch_sync_slow_s);
        } dss = {
                .do_vtable = NULL,
                .dc_func = _dispatch_sync_f_slow2,
                .dc_ctxt = _dispatch_get_thread_semaphore(),
        };

        // XXX FIXME -- concurrent queues can be come serial again
        _dispatch_queue_push(dq, (void *)&dss);

        dispatch_semaphore_wait(dss.dc_ctxt, DISPATCH_TIME_FOREVER);
        _dispatch_put_thread_semaphore(dss.dc_ctxt);
}

#ifdef __BLOCKS__
void
dispatch_sync(dispatch_queue_t dq, void (^work)(void))
{
        struct Block_basic *bb = (void *)work;
        dispatch_sync_f(dq, work, (dispatch_function_t)bb->Block_invoke);
}
#endif

DISPATCH_NOINLINE
void
dispatch_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func)
{
        typeof(dq->dq_running) prev_cnt;
        dispatch_queue_t old_dq;

        if (dq->dq_width == 1) {
                return dispatch_barrier_sync_f(dq, ctxt, func);
        }

        // 1) ensure that this thread hasn't enqueued anything ahead of this call
        // 2) the queue is not suspended
        if (slowpath(dq->dq_items_tail) || slowpath(DISPATCH_OBJECT_SUSPENDED(dq))) {
                _dispatch_sync_f_slow(dq);
        } else {
                prev_cnt = dispatch_atomic_add(&dq->dq_running, 2) - 2;

                if (slowpath(prev_cnt & 1)) {
                        if (dispatch_atomic_sub(&dq->dq_running, 2) == 0) {
                                _dispatch_wakeup(dq);
                        }
                        _dispatch_sync_f_slow(dq);
                }
        }

        old_dq = _dispatch_thread_getspecific(dispatch_queue_key);
        _dispatch_thread_setspecific(dispatch_queue_key, dq);
        func(ctxt);
        _dispatch_workitem_inc();
        _dispatch_thread_setspecific(dispatch_queue_key, old_dq);

        if (slowpath(dispatch_atomic_sub(&dq->dq_running, 2) == 0)) {
                _dispatch_wakeup(dq);
        }
}

const char *
dispatch_queue_get_label(dispatch_queue_t dq)
{
        return dq->dq_label;
}

#if DISPATCH_COCOA_COMPAT
static void
_dispatch_main_q_port_init(void *ctxt __attribute__((unused)))
{
        kern_return_t kr;

        kr = mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &main_q_port);
        DISPATCH_VERIFY_MIG(kr);
        dispatch_assume_zero(kr);
        kr = mach_port_insert_right(mach_task_self(), main_q_port, main_q_port, MACH_MSG_TYPE_MAKE_SEND);
        DISPATCH_VERIFY_MIG(kr);
        dispatch_assume_zero(kr);

        _dispatch_program_is_probably_callback_driven = true;
        _dispatch_safe_fork = false;
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
DISPATCH_NOINLINE
static void
_dispatch_queue_set_mainq_drain_state(bool arg)
{
        main_q_is_draining = arg;
}
#endif

void
dispatch_main(void)
{
        if (pthread_main_np()) {
                _dispatch_program_is_probably_callback_driven = true;
                pthread_exit(NULL);
                DISPATCH_CRASH("pthread_exit() returned");
        }
        DISPATCH_CLIENT_CRASH("dispatch_main() must be called on the main thread");
}

static void
_dispatch_sigsuspend(void *ctxt __attribute__((unused)))
{
        static const sigset_t mask;

        for (;;) {
                sigsuspend(&mask);
        }
}

DISPATCH_NOINLINE
static void
_dispatch_queue_cleanup2(void)
{
        dispatch_atomic_dec(&_dispatch_main_q.dq_running);

        if (dispatch_atomic_sub(&_dispatch_main_q.do_suspend_cnt, DISPATCH_OBJECT_SUSPEND_LOCK) == 0) {
                _dispatch_wakeup(&_dispatch_main_q);
        }

        // overload the "probably" variable to mean that dispatch_main() or
        // similar non-POSIX API was called
        // this has to run before the DISPATCH_COCOA_COMPAT below
        if (_dispatch_program_is_probably_callback_driven) {
                dispatch_async_f(_dispatch_get_root_queue(0, 0), NULL, _dispatch_sigsuspend);
                sleep(1);       // workaround 6778970
        }

#if DISPATCH_COCOA_COMPAT
        dispatch_once_f(&_dispatch_main_q_port_pred, NULL, _dispatch_main_q_port_init);

        mach_port_t mp = main_q_port;
        kern_return_t kr;

        main_q_port = 0;

        if (mp) {
                kr = mach_port_deallocate(mach_task_self(), mp);
                DISPATCH_VERIFY_MIG(kr);
                dispatch_assume_zero(kr);
                kr = mach_port_mod_refs(mach_task_self(), mp, MACH_PORT_RIGHT_RECEIVE, -1);
                DISPATCH_VERIFY_MIG(kr);
                dispatch_assume_zero(kr);
        }
#endif
}

dispatch_queue_t
dispatch_get_concurrent_queue(long pri)
{
        if (pri > 0) {
                pri = DISPATCH_QUEUE_PRIORITY_HIGH;
        } else if (pri < 0) {
                pri = DISPATCH_QUEUE_PRIORITY_LOW;
        }
        return _dispatch_get_root_queue(pri, false);
}

static void
_dispatch_queue_cleanup(void *ctxt)
{
        if (ctxt == &_dispatch_main_q) {
                return _dispatch_queue_cleanup2();
        }
        // POSIX defines that destructors are only called if 'ctxt' is non-null
        DISPATCH_CRASH("Premature thread exit while a dispatch queue is running");
}

dispatch_queue_t
dispatch_get_global_queue(long priority, unsigned long flags)
{
        if (flags & ~DISPATCH_QUEUE_OVERCOMMIT) {
                return NULL;
        }
        return _dispatch_get_root_queue(priority, flags & DISPATCH_QUEUE_OVERCOMMIT);
}

#define countof(x)      (sizeof(x) / sizeof(x[0]))
void
libdispatch_init(void)
{
        dispatch_assert(DISPATCH_QUEUE_PRIORITY_COUNT == 3);
        dispatch_assert(DISPATCH_ROOT_QUEUE_COUNT == 6);

        dispatch_assert(DISPATCH_QUEUE_PRIORITY_LOW == -DISPATCH_QUEUE_PRIORITY_HIGH);
        dispatch_assert(countof(_dispatch_root_queues) == DISPATCH_ROOT_QUEUE_COUNT);
        dispatch_assert(countof(_dispatch_thread_mediator) == DISPATCH_ROOT_QUEUE_COUNT);
        dispatch_assert(countof(_dispatch_root_queue_contexts) == DISPATCH_ROOT_QUEUE_COUNT);

        _dispatch_thread_key_init_np(dispatch_queue_key, _dispatch_queue_cleanup);
        _dispatch_thread_key_init_np(dispatch_sema4_key, (void (*)(void *))dispatch_release);   // use the extern release
        _dispatch_thread_key_init_np(dispatch_cache_key, _dispatch_cache_cleanup2);
#if DISPATCH_PERF_MON
        _dispatch_thread_key_init_np(dispatch_bcounter_key, NULL);
#endif

        _dispatch_thread_setspecific(dispatch_queue_key, &_dispatch_main_q);

        _dispatch_queue_set_width_init();
}

void
_dispatch_queue_unlock(dispatch_queue_t dq)
{
        if (slowpath(dispatch_atomic_dec(&dq->dq_running))) {
                return;
        }

        _dispatch_wakeup(dq);
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
dispatch_queue_t
_dispatch_wakeup(dispatch_object_t dou)
{
        dispatch_queue_t tq;

        if (slowpath(DISPATCH_OBJECT_SUSPENDED(dou._do))) {
                return NULL;
        }
        if (!dx_probe(dou._do) && !dou._dq->dq_items_tail) {
                return NULL;
        }

        if (!_dispatch_trylock(dou._do)) {
#if DISPATCH_COCOA_COMPAT
                if (dou._dq == &_dispatch_main_q) {
                        _dispatch_queue_wakeup_main();
                }
#endif
                return NULL;
        }
        _dispatch_retain(dou._do);
        tq = dou._do->do_targetq;
        _dispatch_queue_push(tq, dou._do);
        return tq;      // libdispatch doesn't need this, but the Instrument DTrace probe does
}

#if DISPATCH_COCOA_COMPAT
DISPATCH_NOINLINE
void
_dispatch_queue_wakeup_main(void)
{
        kern_return_t kr;

        dispatch_once_f(&_dispatch_main_q_port_pred, NULL, _dispatch_main_q_port_init);

        kr = _dispatch_send_wakeup_main_thread(main_q_port, 0);

        switch (kr) {
        case MACH_SEND_TIMEOUT:
        case MACH_SEND_TIMED_OUT:
        case MACH_SEND_INVALID_DEST:
                break;
        default:
                dispatch_assume_zero(kr);
                break;
        }

        _dispatch_safe_fork = false;
}
#endif

static inline int
_dispatch_rootq2wq_pri(long idx)
{
#ifdef WORKQ_DEFAULT_PRIOQUEUE
        switch (idx) {
        case 0:
        case 1:
                return WORKQ_LOW_PRIOQUEUE;
        case 2:
        case 3:
        default:
                return WORKQ_DEFAULT_PRIOQUEUE;
        case 4:
        case 5:
                return WORKQ_HIGH_PRIOQUEUE;
        }
#else
        return pri;
#endif
}

static void
_dispatch_root_queues_init(void *context __attribute__((unused)))
{
        bool disable_wq = getenv("LIBDISPATCH_DISABLE_KWQ");
        pthread_workqueue_attr_t pwq_attr;
        kern_return_t kr;
        int i, r;

        r = pthread_workqueue_attr_init_np(&pwq_attr);
        dispatch_assume_zero(r);

        for (i = 0; i < DISPATCH_ROOT_QUEUE_COUNT; i++) {
                r = pthread_workqueue_attr_setqueuepriority_np(&pwq_attr, _dispatch_rootq2wq_pri(i));
                dispatch_assume_zero(r);
                r = pthread_workqueue_attr_setovercommit_np(&pwq_attr, i & 1);
                dispatch_assume_zero(r);
// some software hangs if the non-overcommitting queues do not overcommit when threads block
#if 0
                if (!(i & 1)) {
                        dispatch_root_queue_contexts[i].dgq_thread_pool_size = _dispatch_hw_config.cc_max_active;
                }
#endif

                r = 0;
                if (disable_wq || (r = pthread_workqueue_create_np(&_dispatch_root_queue_contexts[i].dgq_kworkqueue, &pwq_attr))) {
                        if (r != ENOTSUP) {
                                dispatch_assume_zero(r);
                        }
                        // override the default FIFO behavior for the pool semaphores
                        kr = semaphore_create(mach_task_self(), &_dispatch_thread_mediator[i].dsema_port, SYNC_POLICY_LIFO, 0);
                        DISPATCH_VERIFY_MIG(kr);
                        dispatch_assume_zero(kr);
                        dispatch_assume(_dispatch_thread_mediator[i].dsema_port);
                } else {
                        dispatch_assume(_dispatch_root_queue_contexts[i].dgq_kworkqueue);
                }
        }

        r = pthread_workqueue_attr_destroy_np(&pwq_attr);
        dispatch_assume_zero(r);
}

bool
_dispatch_queue_wakeup_global(dispatch_queue_t dq)
{
        static dispatch_once_t pred;
        struct dispatch_root_queue_context_s *qc = dq->do_ctxt;
        pthread_workitem_handle_t wh;
        unsigned int gen_cnt;
        pthread_t pthr;
        int r, t_count;

        if (!dq->dq_items_tail) {
                return false;
        }

        _dispatch_safe_fork = false;

        dispatch_debug_queue(dq, __PRETTY_FUNCTION__);

        dispatch_once_f(&pred, NULL, _dispatch_root_queues_init);

        if (qc->dgq_kworkqueue) {
                if (dispatch_atomic_cmpxchg(&qc->dgq_pending, 0, 1)) {
                        _dispatch_debug("requesting new worker thread");

                        r = pthread_workqueue_additem_np(qc->dgq_kworkqueue, _dispatch_worker_thread2, dq, &wh, &gen_cnt);
                        dispatch_assume_zero(r);
                } else {
                        _dispatch_debug("work thread request still pending on global queue: %p", dq);
                }
                goto out;
        }

        if (dispatch_semaphore_signal(qc->dgq_thread_mediator)) {
                goto out;
        }

        do {
                t_count = qc->dgq_thread_pool_size;
                if (!t_count) {
                        _dispatch_debug("The thread pool is full: %p", dq);
                        goto out;
                }
        } while (!dispatch_atomic_cmpxchg(&qc->dgq_thread_pool_size, t_count, t_count - 1));

        while ((r = pthread_create(&pthr, NULL, _dispatch_worker_thread, dq))) {
                if (r != EAGAIN) {
                        dispatch_assume_zero(r);
                }
                sleep(1);
        }
        r = pthread_detach(pthr);
        dispatch_assume_zero(r);

out:
        return false;
}

void
_dispatch_queue_serial_drain_till_empty(dispatch_queue_t dq)
{
#if DISPATCH_PERF_MON
        uint64_t start = mach_absolute_time();
#endif
        _dispatch_queue_drain(dq);
#if DISPATCH_PERF_MON
        _dispatch_queue_merge_stats(start);
#endif
        _dispatch_force_cache_cleanup();
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
DISPATCH_NOINLINE
void
_dispatch_queue_invoke(dispatch_queue_t dq)
{
        dispatch_queue_t tq = dq->do_targetq;

        if (!slowpath(DISPATCH_OBJECT_SUSPENDED(dq)) && fastpath(_dispatch_queue_trylock(dq))) {
                _dispatch_queue_drain(dq);
                if (tq == dq->do_targetq) {
                        tq = dx_invoke(dq);
                } else {
                        tq = dq->do_targetq;
                }
                // We do not need to check the result.
                // When the suspend-count lock is dropped, then the check will happen.
                dispatch_atomic_dec(&dq->dq_running);
                if (tq) {
                        return _dispatch_queue_push(tq, dq);
                }
        }

        dq->do_next = DISPATCH_OBJECT_LISTLESS;
        if (dispatch_atomic_sub(&dq->do_suspend_cnt, DISPATCH_OBJECT_SUSPEND_LOCK) == 0) {
                if (dq->dq_running == 0) {
                        _dispatch_wakeup(dq);   // verify that the queue is idle
                }
        }
        _dispatch_release(dq);  // added when the queue is put on the list
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
static void
_dispatch_set_target_queue2(void *ctxt)
{
        dispatch_queue_t prev_dq, dq = _dispatch_queue_get_current();
                  
        prev_dq = dq->do_targetq;
        dq->do_targetq = ctxt;
        _dispatch_release(prev_dq);
}

void
dispatch_set_target_queue(dispatch_object_t dou, dispatch_queue_t dq)
{
        if (slowpath(dou._do->do_xref_cnt == DISPATCH_OBJECT_GLOBAL_REFCNT)) {
                return;
        }
        // NOTE: we test for NULL target queues internally to detect root queues
        // therefore, if the retain crashes due to a bad input, that is OK
        _dispatch_retain(dq);
        dispatch_barrier_async_f(dou._dq, dq, _dispatch_set_target_queue2);
}

static void
_dispatch_async_f_redirect2(void *_ctxt)
{
        struct dispatch_continuation_s *dc = _ctxt;
        struct dispatch_continuation_s *other_dc = dc->dc_data[1];
        dispatch_queue_t old_dq, dq = dc->dc_data[0];

        old_dq = _dispatch_thread_getspecific(dispatch_queue_key);
        _dispatch_thread_setspecific(dispatch_queue_key, dq);
        _dispatch_continuation_pop(other_dc);
        _dispatch_thread_setspecific(dispatch_queue_key, old_dq);

        if (dispatch_atomic_sub(&dq->dq_running, 2) == 0) {
                _dispatch_wakeup(dq);
        }
        _dispatch_release(dq);
}

static void
_dispatch_async_f_redirect(dispatch_queue_t dq, struct dispatch_object_s *other_dc)
{
        dispatch_continuation_t dc = (void *)other_dc;
        dispatch_queue_t root_dq = dq;

        if (dc->dc_func == _dispatch_sync_f_slow2) {
                return dc->dc_func(dc->dc_ctxt);
        }

        dispatch_atomic_add(&dq->dq_running, 2);
        _dispatch_retain(dq);

        dc = _dispatch_continuation_alloc_cacheonly() ?: _dispatch_continuation_alloc_from_heap();

        dc->do_vtable = (void *)DISPATCH_OBJ_ASYNC_BIT;
        dc->dc_func = _dispatch_async_f_redirect2;
        dc->dc_ctxt = dc;
        dc->dc_data[0] = dq;
        dc->dc_data[1] = other_dc;

        do {
                root_dq = root_dq->do_targetq;
        } while (root_dq->do_targetq);

        _dispatch_queue_push(root_dq, dc);
}


void
_dispatch_queue_drain(dispatch_queue_t dq)
{
        dispatch_queue_t orig_tq, old_dq = _dispatch_thread_getspecific(dispatch_queue_key);
        struct dispatch_object_s *dc = NULL, *next_dc = NULL;

        orig_tq = dq->do_targetq;

        _dispatch_thread_setspecific(dispatch_queue_key, dq);

        while (dq->dq_items_tail) {
                while (!fastpath(dq->dq_items_head)) {
                        _dispatch_hardware_pause();
                }

                dc = dq->dq_items_head;
                dq->dq_items_head = NULL;

                do {
                        // Enqueue is TIGHTLY controlled, we won't wait long.
                        do {
                                next_dc = fastpath(dc->do_next);
                        } while (!next_dc && !dispatch_atomic_cmpxchg(&dq->dq_items_tail, dc, NULL));
                        if (DISPATCH_OBJECT_SUSPENDED(dq)) {
                                goto out;
                        }
                        if (dq->dq_running > dq->dq_width) {
                                goto out;
                        }
                        if (orig_tq != dq->do_targetq) {
                                goto out;
                        }
                        if (fastpath(dq->dq_width == 1)) {
                                _dispatch_continuation_pop(dc);
                                _dispatch_workitem_inc();
                        } else if ((long)dc->do_vtable & DISPATCH_OBJ_BARRIER_BIT) {
                                if (dq->dq_running > 1) {
                                        goto out;
                                }
                                _dispatch_continuation_pop(dc);
                                _dispatch_workitem_inc();
                        } else {
                                _dispatch_async_f_redirect(dq, dc);
                        }
                } while ((dc = next_dc));
        }

out:
        // if this is not a complete drain, we must undo some things
        if (slowpath(dc)) {
                // 'dc' must NOT be "popped"
                // 'dc' might be the last item
                if (next_dc || dispatch_atomic_cmpxchg(&dq->dq_items_tail, NULL, dc)) {
                        dq->dq_items_head = dc;
                } else {
                        while (!(next_dc = dq->dq_items_head)) {
                                _dispatch_hardware_pause();
                        }
                        dq->dq_items_head = dc;
                        dc->do_next = next_dc;
                }
        }

        _dispatch_thread_setspecific(dispatch_queue_key, old_dq);
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
void *
_dispatch_worker_thread(void *context)
{
        dispatch_queue_t dq = context;
        struct dispatch_root_queue_context_s *qc = dq->do_ctxt;
        sigset_t mask;
        int r;

        // workaround tweaks the kernel workqueue does for us
        r = sigfillset(&mask);
        dispatch_assume_zero(r);
        r = _dispatch_pthread_sigmask(SIG_BLOCK, &mask, NULL);
        dispatch_assume_zero(r);

        do {
                _dispatch_worker_thread2(context);
                // we use 65 seconds in case there are any timers that run once a minute
        } while (dispatch_semaphore_wait(qc->dgq_thread_mediator, dispatch_time(0, 65ull * NSEC_PER_SEC)) == 0);

        dispatch_atomic_inc(&qc->dgq_thread_pool_size);
        if (dq->dq_items_tail) {
                _dispatch_queue_wakeup_global(dq);
        }

        return NULL;
}

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
void
_dispatch_worker_thread2(void *context)
{
        struct dispatch_object_s *item;
        dispatch_queue_t dq = context;
        struct dispatch_root_queue_context_s *qc = dq->do_ctxt;

        if (_dispatch_thread_getspecific(dispatch_queue_key)) {
                DISPATCH_CRASH("Premature thread recycling");
        }

        _dispatch_thread_setspecific(dispatch_queue_key, dq);
        qc->dgq_pending = 0;

#if DISPATCH_COCOA_COMPAT
        // ensure that high-level memory management techniques do not leak/crash
        dispatch_begin_thread_4GC();
        void *pool = _dispatch_begin_NSAutoReleasePool();
#endif

#if DISPATCH_PERF_MON
        uint64_t start = mach_absolute_time();
#endif
        while ((item = fastpath(_dispatch_queue_concurrent_drain_one(dq)))) {
                _dispatch_continuation_pop(item);
        }
#if DISPATCH_PERF_MON
        _dispatch_queue_merge_stats(start);
#endif

#if DISPATCH_COCOA_COMPAT
        _dispatch_end_NSAutoReleasePool(pool);
        dispatch_end_thread_4GC();
#endif

        _dispatch_thread_setspecific(dispatch_queue_key, NULL);

        _dispatch_force_cache_cleanup();
}

#if DISPATCH_PERF_MON
void
_dispatch_queue_merge_stats(uint64_t start)
{
        uint64_t avg, delta = mach_absolute_time() - start;
        unsigned long count, bucket;

        count = (size_t)_dispatch_thread_getspecific(dispatch_bcounter_key);
        _dispatch_thread_setspecific(dispatch_bcounter_key, NULL);

        if (count) {
                avg = delta / count;
                bucket = flsll(avg);
        } else {
                bucket = 0;
        }

        // 64-bit counters on 32-bit require a lock or a queue
        OSSpinLockLock(&_dispatch_stats_lock);

        _dispatch_stats[bucket].time_total += delta;
        _dispatch_stats[bucket].count_total += count;
        _dispatch_stats[bucket].thread_total++;

        OSSpinLockUnlock(&_dispatch_stats_lock);
}
#endif

size_t
dispatch_queue_debug_attr(dispatch_queue_t dq, char* buf, size_t bufsiz)
{
        return snprintf(buf, bufsiz, "parent = %p ", dq->do_targetq);
}

size_t
dispatch_queue_debug(dispatch_queue_t dq, char* buf, size_t bufsiz)
{
        size_t offset = 0;
        offset += snprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ", dq->dq_label, dq);
        offset += dispatch_object_debug_attr(dq, &buf[offset], bufsiz - offset);
        offset += dispatch_queue_debug_attr(dq, &buf[offset], bufsiz - offset);
        offset += snprintf(&buf[offset], bufsiz - offset, "}");
        return offset;
}

#if DISPATCH_DEBUG
void
dispatch_debug_queue(dispatch_queue_t dq, const char* str) {
        if (fastpath(dq)) {
                dispatch_debug(dq, "%s", str);
        } else {
                _dispatch_log("queue[NULL]: %s", str);
        }
}
#endif

#if DISPATCH_COCOA_COMPAT
void
_dispatch_main_queue_callback_4CF(mach_msg_header_t *msg __attribute__((unused)))
{
        if (main_q_is_draining) {
                return;
        }
        _dispatch_queue_set_mainq_drain_state(true);
        _dispatch_queue_serial_drain_till_empty(&_dispatch_main_q);
        _dispatch_queue_set_mainq_drain_state(false);
}

mach_port_t
_dispatch_get_main_queue_port_4CF(void)
{
        dispatch_once_f(&_dispatch_main_q_port_pred, NULL, _dispatch_main_q_port_init);
        return main_q_port;
}
#endif

static void
dispatch_queue_attr_dispose(dispatch_queue_attr_t attr)
{
        dispatch_queue_attr_set_finalizer(attr, NULL);
        _dispatch_dispose(attr);
}

static const struct dispatch_queue_attr_vtable_s dispatch_queue_attr_vtable = {
        .do_type = DISPATCH_QUEUE_ATTR_TYPE,
        .do_kind = "queue-attr",
        .do_dispose = dispatch_queue_attr_dispose,
};

dispatch_queue_attr_t
dispatch_queue_attr_create(void)
{
        dispatch_queue_attr_t a = calloc(1, sizeof(struct dispatch_queue_attr_s));

        if (a) {
                a->do_vtable = &dispatch_queue_attr_vtable;
                a->do_next = DISPATCH_OBJECT_LISTLESS;
                a->do_ref_cnt = 1;
                a->do_xref_cnt = 1;
                a->do_targetq = _dispatch_get_root_queue(0, 0);
                a->qa_flags = DISPATCH_QUEUE_OVERCOMMIT;
        }
        return a;
}

void
dispatch_queue_attr_set_flags(dispatch_queue_attr_t attr, uint64_t flags)
{
        dispatch_assert_zero(flags & ~DISPATCH_QUEUE_FLAGS_MASK);
        attr->qa_flags = (unsigned long)flags & DISPATCH_QUEUE_FLAGS_MASK;
}
        
void
dispatch_queue_attr_set_priority(dispatch_queue_attr_t attr, int priority)
{
        dispatch_debug_assert(attr, "NULL pointer");
        dispatch_debug_assert(priority <= DISPATCH_QUEUE_PRIORITY_HIGH && priority >= DISPATCH_QUEUE_PRIORITY_LOW, "Invalid priority");

        if (priority > 0) {
                priority = DISPATCH_QUEUE_PRIORITY_HIGH;
        } else if (priority < 0) {
                priority = DISPATCH_QUEUE_PRIORITY_LOW;
        }

        attr->qa_priority = priority;
}

void
dispatch_queue_attr_set_finalizer_f(dispatch_queue_attr_t attr,
        void *context, dispatch_queue_finalizer_function_t finalizer)
{
#ifdef __BLOCKS__
        if (attr->finalizer_func == (void*)_dispatch_call_block_and_release2) {
                Block_release(attr->finalizer_ctxt);
        }
#endif
        attr->finalizer_ctxt = context;
        attr->finalizer_func = finalizer;
}

#ifdef __BLOCKS__
long
dispatch_queue_attr_set_finalizer(dispatch_queue_attr_t attr,
        dispatch_queue_finalizer_t finalizer)
{
        void *ctxt;
        dispatch_queue_finalizer_function_t func;

        if (finalizer) {
                if (!(ctxt = Block_copy(finalizer))) {
                        return 1;
                }
                func = (void *)_dispatch_call_block_and_release2;
        } else {
                ctxt = NULL;
                func = NULL;
        }

        dispatch_queue_attr_set_finalizer_f(attr, ctxt, func);

        return 0;
}
#endif

static void
_dispatch_ccache_init(void *context __attribute__((unused)))
{
        _dispatch_ccache_zone = malloc_create_zone(0, 0);
        dispatch_assert(_dispatch_ccache_zone);
        malloc_set_zone_name(_dispatch_ccache_zone, "DispatchContinuations");
}

dispatch_continuation_t
_dispatch_continuation_alloc_from_heap(void)
{
        static dispatch_once_t pred;
        dispatch_continuation_t dc;

        dispatch_once_f(&pred, NULL, _dispatch_ccache_init);

        while (!(dc = fastpath(malloc_zone_calloc(_dispatch_ccache_zone, 1, ROUND_UP_TO_CACHELINE_SIZE(sizeof(*dc)))))) {
                sleep(1);
        }

        return dc;
}

void
_dispatch_force_cache_cleanup(void)
{
        dispatch_continuation_t dc = _dispatch_thread_getspecific(dispatch_cache_key);
        if (dc) {
                _dispatch_thread_setspecific(dispatch_cache_key, NULL);
                _dispatch_cache_cleanup2(dc);
        }
}

DISPATCH_NOINLINE
void
_dispatch_cache_cleanup2(void *value)
{
        dispatch_continuation_t dc, next_dc = value;

        while ((dc = next_dc)) {
                next_dc = dc->do_next;
                malloc_zone_free(_dispatch_ccache_zone, dc);
        }
}

static char _dispatch_build[16];

static void
_dispatch_bug_init(void *context __attribute__((unused)))
{
        int mib[] = { CTL_KERN, KERN_OSVERSION };
        size_t bufsz = sizeof(_dispatch_build);

        sysctl(mib, 2, _dispatch_build, &bufsz, NULL, 0);
}

void
_dispatch_bug(size_t line, long val)
{
        static dispatch_once_t pred;
        static void *last_seen;
        void *ra = __builtin_return_address(0);

        dispatch_once_f(&pred, NULL, _dispatch_bug_init);
        if (last_seen != ra) {
                last_seen = ra;
                _dispatch_log("BUG in libdispatch: %s - %lu - 0x%lx", _dispatch_build, line, val);
        }
}

void
_dispatch_abort(size_t line, long val)
{
        _dispatch_bug(line, val);
        abort();
}

void
_dispatch_log(const char *msg, ...)
{
        va_list ap;

        va_start(ap, msg);

        _dispatch_logv(msg, ap);

        va_end(ap);
}

void
_dispatch_logv(const char *msg, va_list ap)
{
#if DISPATCH_DEBUG
        static FILE *logfile, *tmp;
        char newbuf[strlen(msg) + 2];
        char path[PATH_MAX];

        sprintf(newbuf, "%s\n", msg);

        if (!logfile) {
                snprintf(path, sizeof(path), "/var/tmp/libdispatch.%d.log", getpid());
                tmp = fopen(path, "a");
                assert(tmp);
                if (!dispatch_atomic_cmpxchg(&logfile, NULL, tmp)) {
                        fclose(tmp);
                } else {
                        struct timeval tv;
                        gettimeofday(&tv, NULL);
                        fprintf(logfile, "=== log file opened for %s[%u] at %ld.%06u ===\n",
                                        getprogname() ?: "", getpid(), tv.tv_sec, tv.tv_usec);
                }
        }
        vfprintf(logfile, newbuf, ap);
        fflush(logfile);
#else
        vsyslog(LOG_NOTICE, msg, ap);
#endif
}

int
_dispatch_pthread_sigmask(int how, sigset_t *set, sigset_t *oset)
{
        int r;

        /* Workaround: 6269619 Not all signals can be delivered on any thread */

        r = sigdelset(set, SIGILL);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGTRAP);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGEMT);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGFPE);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGBUS);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGSEGV);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGSYS);
        dispatch_assume_zero(r);
        r = sigdelset(set, SIGPIPE);
        dispatch_assume_zero(r);

        return pthread_sigmask(how, set, oset);
}

bool _dispatch_safe_fork = true;

void
dispatch_atfork_prepare(void)
{
}

void
dispatch_atfork_parent(void)
{
}

void
dispatch_atfork_child(void)
{
        void *crash = (void *)0x100;
        size_t i;

        if (_dispatch_safe_fork) {
                return;
        }

        _dispatch_main_q.dq_items_head = crash;
        _dispatch_main_q.dq_items_tail = crash;

        _dispatch_mgr_q.dq_items_head = crash;
        _dispatch_mgr_q.dq_items_tail = crash;

        for (i = 0; i < DISPATCH_ROOT_QUEUE_COUNT; i++) {
                _dispatch_root_queues[i].dq_items_head = crash;
                _dispatch_root_queues[i].dq_items_tail = crash;
        }
}

void
dispatch_init_pthread(pthread_t pthr __attribute__((unused)))
{
}

static int _dispatch_kq;

static void
_dispatch_get_kq_init(void *context __attribute__((unused)))
{
        static const struct kevent kev = {
                .ident = 1,
                .filter = EVFILT_USER,
                .flags = EV_ADD|EV_CLEAR,
        };

        _dispatch_kq = kqueue();
        _dispatch_safe_fork = false;
        // in case we fall back to select()
        FD_SET(_dispatch_kq, &_dispatch_rfds);

        if (_dispatch_kq == -1) {
                dispatch_assert_zero(errno);
        }

        dispatch_assume_zero(kevent(_dispatch_kq, &kev, 1, NULL, 0, NULL)); 

        _dispatch_queue_push(_dispatch_mgr_q.do_targetq, &_dispatch_mgr_q);
}

static int
_dispatch_get_kq(void)
{
        static dispatch_once_t pred;

        dispatch_once_f(&pred, NULL, _dispatch_get_kq_init);

        return _dispatch_kq;
}

static void
_dispatch_mgr_thread2(struct kevent *kev, size_t cnt)
{
        size_t i;

        for (i = 0; i < cnt; i++) {
                // EVFILT_USER isn't used by sources
                if (kev[i].filter == EVFILT_USER) {
                        // If _dispatch_mgr_thread2() ever is changed to return to the
                        // caller, then this should become _dispatch_queue_drain()
                        _dispatch_queue_serial_drain_till_empty(&_dispatch_mgr_q);
                } else {
                        _dispatch_source_drain_kevent(&kev[i]);
                }
    }
}

static dispatch_queue_t
_dispatch_mgr_invoke(dispatch_queue_t dq)
{
        static const struct timespec timeout_immediately = { 0, 0 };
        struct timespec timeout;
        const struct timespec *timeoutp;
        struct timeval sel_timeout, *sel_timeoutp;
        fd_set tmp_rfds, tmp_wfds;
        struct kevent kev[1];
        int k_cnt, k_err, i, r;

        _dispatch_thread_setspecific(dispatch_queue_key, dq);

        for (;;) {
                _dispatch_run_timers();

                timeoutp = _dispatch_get_next_timer_fire(&timeout);
                
                if (_dispatch_select_workaround) {
                        FD_COPY(&_dispatch_rfds, &tmp_rfds);
                        FD_COPY(&_dispatch_wfds, &tmp_wfds);
                        if (timeoutp) {
                                sel_timeout.tv_sec = timeoutp->tv_sec;
                                sel_timeout.tv_usec = (typeof(sel_timeout.tv_usec))(timeoutp->tv_nsec / 1000u);
                                sel_timeoutp = &sel_timeout;
                        } else {
                                sel_timeoutp = NULL;
                        }
                        
                        r = select(FD_SETSIZE, &tmp_rfds, &tmp_wfds, NULL, sel_timeoutp);
                        if (r == -1) {
                                if (errno != EBADF) {
                                        dispatch_assume_zero(errno);
                                        continue;
                                }
                                for (i = 0; i < FD_SETSIZE; i++) {
                                        if (i == _dispatch_kq) {
                                                continue;
                                        }
                                        if (!FD_ISSET(i, &_dispatch_rfds) && !FD_ISSET(i, &_dispatch_wfds)) {
                                                continue;
                                        }
                                        r = dup(i);
                                        if (r != -1) {
                                                close(r);
                                        } else {
                                                FD_CLR(i, &_dispatch_rfds);
                                                FD_CLR(i, &_dispatch_wfds);
                                                _dispatch_rfd_ptrs[i] = 0;
                                                _dispatch_wfd_ptrs[i] = 0;
                                        }
                                }
                                continue;
                        }
                        
                        if (r > 0) {
                                for (i = 0; i < FD_SETSIZE; i++) {
                                        if (i == _dispatch_kq) {
                                                continue;
                                        }
                                        if (FD_ISSET(i, &tmp_rfds)) {
                                                FD_CLR(i, &_dispatch_rfds);     // emulate EV_DISABLE
                                                EV_SET(&kev[0], i, EVFILT_READ, EV_ADD|EV_ENABLE|EV_DISPATCH, 0, 1, _dispatch_rfd_ptrs[i]);
                                                _dispatch_rfd_ptrs[i] = 0;
                                                _dispatch_mgr_thread2(kev, 1);
                                        }
                                        if (FD_ISSET(i, &tmp_wfds)) {
                                                FD_CLR(i, &_dispatch_wfds);     // emulate EV_DISABLE
                                                EV_SET(&kev[0], i, EVFILT_WRITE, EV_ADD|EV_ENABLE|EV_DISPATCH, 0, 1, _dispatch_wfd_ptrs[i]);
                                                _dispatch_wfd_ptrs[i] = 0;
                                                _dispatch_mgr_thread2(kev, 1);
                                        }
                                }
                        }
                        
                        timeoutp = &timeout_immediately;
                }
                
                k_cnt = kevent(_dispatch_kq, NULL, 0, kev, sizeof(kev) / sizeof(kev[0]), timeoutp);
                k_err = errno;

                switch (k_cnt) {
                case -1:
                        if (k_err == EBADF) {
                                DISPATCH_CLIENT_CRASH("Do not close random Unix descriptors");
                        }
                        dispatch_assume_zero(k_err);
                        continue;
                default:
                        _dispatch_mgr_thread2(kev, (size_t)k_cnt);
                        // fall through
                case 0:
                        _dispatch_force_cache_cleanup();
                        continue;
                }
        }

        return NULL;
}

static bool
_dispatch_mgr_wakeup(dispatch_queue_t dq)
{
        static const struct kevent kev = {
                .ident = 1,
                .filter = EVFILT_USER,
#ifdef EV_TRIGGER
                .flags = EV_TRIGGER,
#endif
#ifdef NOTE_TRIGGER
                .fflags = NOTE_TRIGGER,
#endif
        };

        _dispatch_debug("waking up the _dispatch_mgr_q: %p", dq);

        _dispatch_update_kq(&kev);

        return false;
}

void
_dispatch_update_kq(const struct kevent *kev)
{
        struct kevent kev_copy = *kev;
        kev_copy.flags |= EV_RECEIPT;

        if (kev_copy.flags & EV_DELETE) {
                switch (kev_copy.filter) {
                case EVFILT_READ:
                        if (FD_ISSET((int)kev_copy.ident, &_dispatch_rfds)) {
                                FD_CLR((int)kev_copy.ident, &_dispatch_rfds);
                                _dispatch_rfd_ptrs[kev_copy.ident] = 0;
                                return;
                        }
                case EVFILT_WRITE:
                        if (FD_ISSET((int)kev_copy.ident, &_dispatch_wfds)) {
                                FD_CLR((int)kev_copy.ident, &_dispatch_wfds);
                                _dispatch_wfd_ptrs[kev_copy.ident] = 0;
                                return;
                        }
                default:
                        break;
                }
        }
        
        int rval = kevent(_dispatch_get_kq(), &kev_copy, 1, &kev_copy, 1, NULL);
        if (rval == -1) { 
                // If we fail to register with kevents, for other reasons aside from
                // changelist elements.
                dispatch_assume_zero(errno);
                //kev_copy.flags |= EV_ERROR;
                //kev_copy.data = error;
                return;
        }

        // The following select workaround only applies to adding kevents
        if (!(kev->flags & EV_ADD)) {
                return;
        }

        switch (kev_copy.data) {
        case 0:
                return;
        case EBADF:
                break;
        default:
                // If an error occurred while registering with kevent, and it was 
                // because of a kevent changelist processing && the kevent involved
                // either doing a read or write, it would indicate we were trying
                // to register a /dev/* port; fall back to select
                switch (kev_copy.filter) {
                case EVFILT_READ:
                        _dispatch_select_workaround = true;
                        FD_SET((int)kev_copy.ident, &_dispatch_rfds);
                        _dispatch_rfd_ptrs[kev_copy.ident] = kev_copy.udata;
                        break;
                case EVFILT_WRITE:
                        _dispatch_select_workaround = true;
                        FD_SET((int)kev_copy.ident, &_dispatch_wfds);
                        _dispatch_wfd_ptrs[kev_copy.ident] = kev_copy.udata;
                        break;
                default:
                        _dispatch_source_drain_kevent(&kev_copy); 
                        break;
                }
                break;
        }
}

static const struct dispatch_queue_vtable_s _dispatch_queue_mgr_vtable = {
        .do_type = DISPATCH_QUEUE_MGR_TYPE,
        .do_kind = "mgr-queue",
        .do_invoke = _dispatch_mgr_invoke,
        .do_debug = dispatch_queue_debug,
        .do_probe = _dispatch_mgr_wakeup,
};

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
struct dispatch_queue_s _dispatch_mgr_q = {
        .do_vtable = &_dispatch_queue_mgr_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_targetq = &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_COUNT - 1],

        .dq_label = "com.apple.libdispatch-manager",
        .dq_width = 1,
        .dq_serialnum = 2,
};

const struct dispatch_queue_offsets_s dispatch_queue_offsets = {
        .dqo_version = 3,
        .dqo_label = offsetof(struct dispatch_queue_s, dq_label),
        .dqo_label_size = sizeof(_dispatch_main_q.dq_label),
        .dqo_flags = 0,
        .dqo_flags_size = 0,
        .dqo_width = offsetof(struct dispatch_queue_s, dq_width),
        .dqo_width_size = sizeof(_dispatch_main_q.dq_width),
        .dqo_serialnum = offsetof(struct dispatch_queue_s, dq_serialnum),
        .dqo_serialnum_size = sizeof(_dispatch_main_q.dq_serialnum),
        .dqo_running = offsetof(struct dispatch_queue_s, dq_running),
        .dqo_running_size = sizeof(_dispatch_main_q.dq_running),
};

#ifdef __BLOCKS__
void
dispatch_after(dispatch_time_t when, dispatch_queue_t queue, dispatch_block_t work)
{
        // test before the copy of the block
        if (when == DISPATCH_TIME_FOREVER) {
#if DISPATCH_DEBUG
                DISPATCH_CLIENT_CRASH("dispatch_after() called with 'when' == infinity");
#endif
                return;
        }
        dispatch_after_f(when, queue, _dispatch_Block_copy(work), _dispatch_call_block_and_release);
}
#endif

DISPATCH_NOINLINE
void
dispatch_after_f(dispatch_time_t when, dispatch_queue_t queue, void *ctxt, void (*func)(void *))
{
        uint64_t delta;
        if (when == DISPATCH_TIME_FOREVER) {
#if DISPATCH_DEBUG
                DISPATCH_CLIENT_CRASH("dispatch_after_f() called with 'when' == infinity");
#endif
                return;
        }

        // this function can and should be optimized to not use a dispatch source
again:
        delta = _dispatch_timeout(when);
        if (delta == 0) {
                return dispatch_async_f(queue, ctxt, func);
        }
        if (!dispatch_source_timer_create(DISPATCH_TIMER_INTERVAL, delta, 0, NULL, queue, ^(dispatch_source_t ds) {
                long err_dom, err_val;
                if ((err_dom = dispatch_source_get_error(ds, &err_val))) {
                        dispatch_assert(err_dom == DISPATCH_ERROR_DOMAIN_POSIX);
                        dispatch_assert(err_val == ECANCELED);
                        func(ctxt);
                        dispatch_release(ds);   // MUST NOT be _dispatch_release()
                } else {
                        dispatch_source_cancel(ds);
                }
        })) {
                goto again;
        }
}