libdispatch-84.5.5.tar.gz

[apple/libdispatch.git] / src / semaphore.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"

// semaphores are too fundamental to use the dispatch_assume*() macros
#define DISPATCH_SEMAPHORE_VERIFY_KR(x) do {    \
                if (x) {        \
                        DISPATCH_CRASH("flawed group/semaphore logic"); \
                }       \
        } while (0)

struct dispatch_semaphore_vtable_s {
        DISPATCH_VTABLE_HEADER(dispatch_semaphore_s);
};

static void _dispatch_semaphore_dispose(dispatch_semaphore_t dsema);
static size_t _dispatch_semaphore_debug(dispatch_semaphore_t dsema, char *buf, size_t bufsiz);
static long _dispatch_group_wake(dispatch_semaphore_t dsema);

const struct dispatch_semaphore_vtable_s _dispatch_semaphore_vtable = {
        .do_type = DISPATCH_SEMAPHORE_TYPE,
        .do_kind = "semaphore",
        .do_dispose = _dispatch_semaphore_dispose,
        .do_debug = _dispatch_semaphore_debug,
};

dispatch_semaphore_t
_dispatch_get_thread_semaphore(void)
{
        dispatch_semaphore_t dsema;
        
        dsema = fastpath(_dispatch_thread_getspecific(dispatch_sema4_key));
        if (!dsema) {
                while (!(dsema = dispatch_semaphore_create(0))) {
                        sleep(1);
                }
        }
        _dispatch_thread_setspecific(dispatch_sema4_key, NULL);
        return dsema;
}

void
_dispatch_put_thread_semaphore(dispatch_semaphore_t dsema)
{
        dispatch_semaphore_t old_sema = _dispatch_thread_getspecific(dispatch_sema4_key);
        _dispatch_thread_setspecific(dispatch_sema4_key, dsema);
        if (old_sema) {
                dispatch_release(old_sema);
        }
}

dispatch_group_t
dispatch_group_create(void)
{
        return (dispatch_group_t)dispatch_semaphore_create(LONG_MAX);
}

dispatch_semaphore_t
dispatch_semaphore_create(long value)
{
        dispatch_semaphore_t dsema;
        
        // If the internal value is negative, then the absolute of the value is
        // equal to the number of waiting threads. Therefore it is bogus to
        // initialize the semaphore with a negative value.
        if (value < 0) {
                return NULL;
        }
        
        dsema = calloc(1, sizeof(struct dispatch_semaphore_s));
        
        if (fastpath(dsema)) {
                dsema->do_vtable = &_dispatch_semaphore_vtable;
                dsema->do_next = DISPATCH_OBJECT_LISTLESS;
                dsema->do_ref_cnt = 1;
                dsema->do_xref_cnt = 1;
                dsema->do_targetq = dispatch_get_global_queue(0, 0);
                dsema->dsema_value = value;
                dsema->dsema_orig = value;
        }
        
        return dsema;
}

static void
_dispatch_semaphore_create_port(semaphore_t *s4)
{
        kern_return_t kr;
        semaphore_t tmp;

        if (*s4) {
                return;
        }
        
        // lazily allocate the semaphore port
        
        // Someday:
        // 1) Switch to a doubly-linked FIFO in user-space.
        // 2) User-space timers for the timeout.
        // 3) Use the per-thread semaphore port.
        
        while (dispatch_assume_zero(kr = semaphore_create(mach_task_self(), &tmp, SYNC_POLICY_FIFO, 0))) {
                DISPATCH_VERIFY_MIG(kr);
                sleep(1);
        }
        
        if (!dispatch_atomic_cmpxchg(s4, 0, tmp)) {
                kr = semaphore_destroy(mach_task_self(), tmp);
                DISPATCH_SEMAPHORE_VERIFY_KR(kr);
        }

        _dispatch_safe_fork = false;
}

DISPATCH_NOINLINE
static long
_dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
        mach_timespec_t _timeout;
        kern_return_t kr;
        uint64_t nsec;
        long orig;
        
again:
        // Mach semaphores appear to sometimes spuriously wake up.  Therefore,
        // we keep a parallel count of the number of times a Mach semaphore is
        // signaled.
        while ((orig = dsema->dsema_sent_ksignals)) {
                if (dispatch_atomic_cmpxchg(&dsema->dsema_sent_ksignals, orig, orig - 1)) {
                        return 0;
                }
        }

        _dispatch_semaphore_create_port(&dsema->dsema_port);

        // From xnu/osfmk/kern/sync_sema.c:
        // wait_semaphore->count = -1;  /* we don't keep an actual count */
        //
        // The code above does not match the documentation, and that fact is
        // not surprising. The documented semantics are clumsy to use in any
        // practical way. The above hack effectively tricks the rest of the
        // Mach semaphore logic to behave like the libdispatch algorithm.

        switch (timeout) {
        default:
                do {
                        // timeout() already calculates relative time left
                        nsec = _dispatch_timeout(timeout);
                        _timeout.tv_sec = (typeof(_timeout.tv_sec))(nsec / NSEC_PER_SEC);
                        _timeout.tv_nsec = (typeof(_timeout.tv_nsec))(nsec % NSEC_PER_SEC);
                        kr = slowpath(semaphore_timedwait(dsema->dsema_port, _timeout));
                } while (kr == KERN_ABORTED);

                if (kr != KERN_OPERATION_TIMED_OUT) {
                        DISPATCH_SEMAPHORE_VERIFY_KR(kr);
                        break;
                }
                // Fall through and try to undo what the fast path did to dsema->dsema_value
        case DISPATCH_TIME_NOW:
                while ((orig = dsema->dsema_value) < 0) {
                        if (dispatch_atomic_cmpxchg(&dsema->dsema_value, orig, orig + 1)) {
                                return KERN_OPERATION_TIMED_OUT;
                        }
                }
                // Another thread called semaphore_signal().
                // Fall through and drain the wakeup.
        case DISPATCH_TIME_FOREVER:
                do {
                        kr = semaphore_wait(dsema->dsema_port);
                } while (kr == KERN_ABORTED);
                DISPATCH_SEMAPHORE_VERIFY_KR(kr);
                break;
        }

        goto again;
}

DISPATCH_NOINLINE
void
dispatch_group_enter(dispatch_group_t dg)
{
        dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;
#if defined(__OPTIMIZE__) && defined(__GNUC__) && (defined(__x86_64__) || defined(__i386__))
        // This assumes:
        // 1) Way too much about the optimizer of GCC.
        // 2) There will never be more than LONG_MAX threads.
        //    Therefore: no overflow detection
        asm(
#ifdef __LP64__ 
                "lock decq      %0\n\t"
#else
                "lock decl      %0\n\t"
#endif
                "js     1f\n\t"
                "xor    %%eax, %%eax\n\t"
                "ret\n\t"
                "1:"
                : "+m" (dsema->dsema_value)
                :
                : "cc"
        );
        _dispatch_semaphore_wait_slow(dsema, DISPATCH_TIME_FOREVER);
#else
        dispatch_semaphore_wait(dsema, DISPATCH_TIME_FOREVER);
#endif
}

DISPATCH_NOINLINE
long
dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
#if defined(__OPTIMIZE__) && defined(__GNUC__) && (defined(__x86_64__) || defined(__i386__))
        // This assumes:
        // 1) Way too much about the optimizer of GCC.
        // 2) There will never be more than LONG_MAX threads.
        //    Therefore: no overflow detection
        asm(
#ifdef __LP64__ 
                "lock decq      %0\n\t"
#else
                "lock decl      %0\n\t"
#endif
                "js     1f\n\t"
                "xor    %%eax, %%eax\n\t"
                "ret\n\t"
                "1:"
                : "+m" (dsema->dsema_value)
                :
                : "cc"
        );
#else
        if (dispatch_atomic_dec(&dsema->dsema_value) >= 0) {
                return 0;
        }
#endif
        return _dispatch_semaphore_wait_slow(dsema, timeout);
}

DISPATCH_NOINLINE
static long
_dispatch_semaphore_signal_slow(dispatch_semaphore_t dsema)
{
        kern_return_t kr;
        
        _dispatch_semaphore_create_port(&dsema->dsema_port);

        // Before dsema_sent_ksignals is incremented we can rely on the reference
        // held by the waiter. However, once this value is incremented the waiter
        // may return between the atomic increment and the semaphore_signal(),
        // therefore an explicit reference must be held in order to safely access
        // dsema after the atomic increment.
        _dispatch_retain(dsema);
        
        dispatch_atomic_inc(&dsema->dsema_sent_ksignals);
        
        kr = semaphore_signal(dsema->dsema_port);
        DISPATCH_SEMAPHORE_VERIFY_KR(kr);

        _dispatch_release(dsema);
        
        return 1;
}

void
dispatch_group_leave(dispatch_group_t dg)
{
        dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;

        dispatch_semaphore_signal(dsema);

        if (dsema->dsema_value == dsema->dsema_orig) {
                _dispatch_group_wake(dsema);
        }
}

DISPATCH_NOINLINE
long
dispatch_semaphore_signal(dispatch_semaphore_t dsema)
{
#if defined(__OPTIMIZE__) && defined(__GNUC__) && (defined(__x86_64__) || defined(__i386__))
        // overflow detection
        // this assumes way too much about the optimizer of GCC
        asm(
#ifdef __LP64__ 
                "lock incq      %0\n\t"
#else
                "lock incl      %0\n\t"
#endif
                "jo     1f\n\t"
                "jle    2f\n\t"
                "xor    %%eax, %%eax\n\t"
                "ret\n\t"
                "1:\n\t"
                "int    $4\n\t"
                "2:"
                : "+m" (dsema->dsema_value)
                :
                : "cc"
        );
#else
        if (dispatch_atomic_inc(&dsema->dsema_value) > 0) {
                return 0;
        }
#endif
        return _dispatch_semaphore_signal_slow(dsema);
}

DISPATCH_NOINLINE
long
_dispatch_group_wake(dispatch_semaphore_t dsema)
{
        struct dispatch_sema_notify_s *tmp, *head = dispatch_atomic_xchg(&dsema->dsema_notify_head, NULL);
        long rval = dispatch_atomic_xchg(&dsema->dsema_group_waiters, 0);
        bool do_rel = head;
        long kr;

        // wake any "group" waiter or notify blocks
        
        if (rval) {
                _dispatch_semaphore_create_port(&dsema->dsema_waiter_port);
                do {
                        kr = semaphore_signal(dsema->dsema_waiter_port);
                        DISPATCH_SEMAPHORE_VERIFY_KR(kr);
                } while (--rval);
        }
        while (head) {
                dispatch_async_f(head->dsn_queue, head->dsn_ctxt, head->dsn_func);
                _dispatch_release(head->dsn_queue);
                do {
                        tmp = head->dsn_next;
                } while (!tmp && !dispatch_atomic_cmpxchg(&dsema->dsema_notify_tail, head, NULL));
                free(head);
                head = tmp;
        }
        if (do_rel) {
                _dispatch_release(dsema);
        }
        return 0;
}

DISPATCH_NOINLINE
static long
_dispatch_group_wait_slow(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
        mach_timespec_t _timeout;
        kern_return_t kr;
        uint64_t nsec;
        long orig;
        
again:
        // check before we cause another signal to be sent by incrementing dsema->dsema_group_waiters
        if (dsema->dsema_value == dsema->dsema_orig) {
                return _dispatch_group_wake(dsema);
        }
        // Mach semaphores appear to sometimes spuriously wake up.  Therefore,
        // we keep a parallel count of the number of times a Mach semaphore is
        // signaled.
        dispatch_atomic_inc(&dsema->dsema_group_waiters);
        // check the values again in case we need to wake any threads
        if (dsema->dsema_value == dsema->dsema_orig) {
                return _dispatch_group_wake(dsema);
        }

        _dispatch_semaphore_create_port(&dsema->dsema_waiter_port);
        
        // From xnu/osfmk/kern/sync_sema.c:
        // wait_semaphore->count = -1;  /* we don't keep an actual count */
        //
        // The code above does not match the documentation, and that fact is
        // not surprising. The documented semantics are clumsy to use in any
        // practical way. The above hack effectively tricks the rest of the
        // Mach semaphore logic to behave like the libdispatch algorithm.
        
        switch (timeout) {
        default:
                do {
                        nsec = _dispatch_timeout(timeout);
                        _timeout.tv_sec = (typeof(_timeout.tv_sec))(nsec / NSEC_PER_SEC);
                        _timeout.tv_nsec = (typeof(_timeout.tv_nsec))(nsec % NSEC_PER_SEC);
                        kr = slowpath(semaphore_timedwait(dsema->dsema_waiter_port, _timeout));
                } while (kr == KERN_ABORTED);
                if (kr != KERN_OPERATION_TIMED_OUT) {
                        DISPATCH_SEMAPHORE_VERIFY_KR(kr);
                        break;
                }
                // Fall through and try to undo the earlier change to dsema->dsema_group_waiters
        case DISPATCH_TIME_NOW:
                while ((orig = dsema->dsema_group_waiters)) {
                        if (dispatch_atomic_cmpxchg(&dsema->dsema_group_waiters, orig, orig - 1)) {
                                return KERN_OPERATION_TIMED_OUT;
                        }
                }
                // Another thread called semaphore_signal().
                // Fall through and drain the wakeup.
        case DISPATCH_TIME_FOREVER:
                do {
                        kr = semaphore_wait(dsema->dsema_waiter_port);
                } while (kr == KERN_ABORTED);
                DISPATCH_SEMAPHORE_VERIFY_KR(kr);
                break;
        }

        goto again;
}

long
dispatch_group_wait(dispatch_group_t dg, dispatch_time_t timeout)
{
        dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;

        if (dsema->dsema_value == dsema->dsema_orig) {
                return 0;
        }
        if (timeout == 0) {
                return KERN_OPERATION_TIMED_OUT;
        }
        return _dispatch_group_wait_slow(dsema, timeout);
}

#ifdef __BLOCKS__
void
dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq, dispatch_block_t db)
{
        dispatch_group_notify_f(dg, dq, _dispatch_Block_copy(db), _dispatch_call_block_and_release);
}
#endif

void
dispatch_group_notify_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt, void (*func)(void *))
{
        dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;
        struct dispatch_sema_notify_s *dsn, *prev;

        // FIXME -- this should be updated to use the continuation cache
        while (!(dsn = malloc(sizeof(*dsn)))) {
                sleep(1);
        }

        dsn->dsn_next = NULL;
        dsn->dsn_queue = dq;
        dsn->dsn_ctxt = ctxt;
        dsn->dsn_func = func;
        _dispatch_retain(dq);

        prev = dispatch_atomic_xchg(&dsema->dsema_notify_tail, dsn);
        if (fastpath(prev)) {
                prev->dsn_next = dsn;
        } else {
                _dispatch_retain(dg);
                dsema->dsema_notify_head = dsn;
                if (dsema->dsema_value == dsema->dsema_orig) {
                        _dispatch_group_wake(dsema);
                }
        }
}

void
_dispatch_semaphore_dispose(dispatch_semaphore_t dsema)
{
        kern_return_t kr;
        
        if (dsema->dsema_value < dsema->dsema_orig) {
                DISPATCH_CLIENT_CRASH("Semaphore/group object deallocated while in use");
        }
        
        if (dsema->dsema_port) {
                kr = semaphore_destroy(mach_task_self(), dsema->dsema_port);
                DISPATCH_SEMAPHORE_VERIFY_KR(kr);
        }
        if (dsema->dsema_waiter_port) {
                kr = semaphore_destroy(mach_task_self(), dsema->dsema_waiter_port);
                DISPATCH_SEMAPHORE_VERIFY_KR(kr);
        }
        
        _dispatch_dispose(dsema);
}

size_t
_dispatch_semaphore_debug(dispatch_semaphore_t dsema, char *buf, size_t bufsiz)
{
        size_t offset = 0;
        offset += snprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ", dx_kind(dsema), dsema);
        offset += dispatch_object_debug_attr(dsema, &buf[offset], bufsiz - offset);
        offset += snprintf(&buf[offset], bufsiz - offset, "port = 0x%u, value = %ld, orig = %ld }",
                                           dsema->dsema_port, dsema->dsema_value, dsema->dsema_orig);
        return offset;
}

#ifdef __BLOCKS__
void
dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq, dispatch_block_t db)
{
        dispatch_group_async_f(dg, dq, _dispatch_Block_copy(db), _dispatch_call_block_and_release);
}
#endif

DISPATCH_NOINLINE
void
dispatch_group_async_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt, void (*func)(void *))
{
        dispatch_continuation_t dc;

        _dispatch_retain(dg);
        dispatch_group_enter(dg);

        dc = _dispatch_continuation_alloc_cacheonly() ?: _dispatch_continuation_alloc_from_heap();

        dc->do_vtable = (void *)(DISPATCH_OBJ_ASYNC_BIT|DISPATCH_OBJ_GROUP_BIT);
        dc->dc_func = func;
        dc->dc_ctxt = ctxt;
        dc->dc_group = dg;

        _dispatch_queue_push(dq, dc);
}