src/shims/lock.c - third_party/swift-corelibs-libdispatch - Git at Google

 /*
  * Copyright (c) 2016 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"

 #define _dlock_syscall_switch(err, syscall, ...) \
 	for (;;) { \
 		int err; \
 		switch ((err = ((syscall) < 0 ? errno : 0))) { \
 		case EINTR: continue; \
 		__VA_ARGS__ \
 		} \
 		break; \
 	}

 #if TARGET_OS_MAC
 _Static_assert(DLOCK_LOCK_DATA_CONTENTION == ULF_WAIT_WORKQ_DATA_CONTENTION,
 		"values should be the same");

 DISPATCH_ALWAYS_INLINE
 static inline void
 _dispatch_thread_switch(dispatch_lock value, dispatch_lock_options_t flags,
 		uint32_t timeout)
 {
 	int option;
 	if (flags & DLOCK_LOCK_DATA_CONTENTION) {
 		option = SWITCH_OPTION_OSLOCK_DEPRESS;
 	} else {
 		option = SWITCH_OPTION_DEPRESS;
 	}
 	thread_switch(_dispatch_lock_owner(value), option, timeout);
 }
 #endif

 #pragma mark - ulock wrappers
 #if HAVE_UL_COMPARE_AND_WAIT

 static int
 _dispatch_ulock_wait(uint32_t *uaddr, uint32_t val, uint32_t timeout,
 		uint32_t flags)
 {
 	dispatch_assert(!DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK);
 	int rc;
 	_dlock_syscall_switch(err,
 		rc = __ulock_wait(UL_COMPARE_AND_WAIT | flags, uaddr, val, timeout),
 		case 0: return rc > 0 ? ENOTEMPTY : 0;
 		case ETIMEDOUT: case EFAULT: return err;
 		default: DISPATCH_INTERNAL_CRASH(err, "ulock_wait() failed");
 	);
 }

 static void
 _dispatch_ulock_wake(uint32_t *uaddr, uint32_t flags)
 {
 	dispatch_assert(!DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK);
 	_dlock_syscall_switch(err,
 		__ulock_wake(UL_COMPARE_AND_WAIT | flags, uaddr, 0),
 		case 0: case ENOENT: break;
 		default: DISPATCH_INTERNAL_CRASH(err, "ulock_wake() failed");
 	);
 }

 #endif
 #if HAVE_UL_UNFAIR_LOCK

 // returns 0, ETIMEDOUT, ENOTEMPTY, EFAULT
 static int
 _dispatch_unfair_lock_wait(uint32_t *uaddr, uint32_t val, uint32_t timeout,
 		dispatch_lock_options_t flags)
 {
 	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
 		// <rdar://problem/25075359>
 		timeout =  timeout < 1000 ? 1 : timeout / 1000;
 		_dispatch_thread_switch(val, flags, timeout);
 		return 0;
 	}
 	int rc;
 	_dlock_syscall_switch(err,
 		rc = __ulock_wait(UL_UNFAIR_LOCK | flags, uaddr, val, timeout),
 		case 0: return rc > 0 ? ENOTEMPTY : 0;
 		case ETIMEDOUT: case EFAULT: return err;
 		default: DISPATCH_INTERNAL_CRASH(err, "ulock_wait() failed");
 	);
 }

 static void
 _dispatch_unfair_lock_wake(uint32_t *uaddr, uint32_t flags)
 {
 	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
 		// <rdar://problem/25075359>
 		return;
 	}
 	_dlock_syscall_switch(err, __ulock_wake(UL_UNFAIR_LOCK | flags, uaddr, 0),
 		case 0: case ENOENT: break;
 		default: DISPATCH_INTERNAL_CRASH(err, "ulock_wake() failed");
 	);
 }

 #endif
 #pragma mark - futex wrappers
 #if HAVE_FUTEX
 #include <sys/time.h>
 #include <syscall.h>

 DISPATCH_ALWAYS_INLINE
 static inline int
 _dispatch_futex(uint32_t *uaddr, int op, uint32_t val,
 		const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3,
 		int opflags)
 {
 	return syscall(SYS_futex, uaddr, op | opflags, val, timeout, uaddr2, val3);
 }

 static int
 _dispatch_futex_wait(uint32_t *uaddr, uint32_t val,
 		const struct timespec *timeout, int opflags)
 {
 	_dlock_syscall_switch(err,
 		_dispatch_futex(uaddr, FUTEX_WAIT, val, timeout, NULL, 0, opflags),
 		case 0: case EWOULDBLOCK: case ETIMEDOUT: return err;
 		default: DISPATCH_CLIENT_CRASH(err, "futex_wait() failed");
 	);
 }

 static void
 _dispatch_futex_wake(uint32_t *uaddr, int wake, int opflags)
 {
 	int rc;
 	_dlock_syscall_switch(err,
 		rc = _dispatch_futex(uaddr, FUTEX_WAKE, wake, NULL, NULL, 0, opflags),
 		case 0: return;
 		default: DISPATCH_CLIENT_CRASH(err, "futex_wake() failed");
 	);
 }

 static void
 _dispatch_futex_lock_pi(uint32_t *uaddr, struct timespec *timeout, int detect,
 	      int opflags)
 {
 	_dlock_syscall_switch(err,
 		_dispatch_futex(uaddr, FUTEX_LOCK_PI, detect, timeout,
 				NULL, 0, opflags),
 		case 0: return;
 		default: DISPATCH_CLIENT_CRASH(errno, "futex_lock_pi() failed");
 	);
 }

 static void
 _dispatch_futex_unlock_pi(uint32_t *uaddr, int opflags)
 {
 	_dlock_syscall_switch(err,
 		_dispatch_futex(uaddr, FUTEX_UNLOCK_PI, 0, NULL, NULL, 0, opflags),
 		case 0: return;
 		default: DISPATCH_CLIENT_CRASH(errno, "futex_unlock_pi() failed");
 	);
 }

 #endif
 #pragma mark - wait for address

 void
 _dispatch_wait_on_address(uint32_t volatile *address, uint32_t value,
 		dispatch_lock_options_t flags)
 {
 #if HAVE_UL_COMPARE_AND_WAIT
 	_dispatch_ulock_wait((uint32_t *)address, value, 0, flags);
 #elif HAVE_FUTEX
 	_dispatch_futex_wait((uint32_t *)address, value, NULL, FUTEX_PRIVATE_FLAG);
 #else
 	mach_msg_timeout_t timeout = 1;
 	while (os_atomic_load(address, relaxed) == value) {
 		thread_switch(MACH_PORT_NULL, SWITCH_OPTION_WAIT, timeout++);
 	}
 #endif
 	(void)flags;
 }

 void
 _dispatch_wake_by_address(uint32_t volatile *address)
 {
 #if HAVE_UL_COMPARE_AND_WAIT
 	_dispatch_ulock_wake((uint32_t *)address, ULF_WAKE_ALL);
 #elif HAVE_FUTEX
 	_dispatch_futex_wake((uint32_t *)address, INT_MAX, FUTEX_PRIVATE_FLAG);
 #else
 	(void)address;
 #endif
 }

 #pragma mark - thread event

 #if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
 semaphore_t
 _dispatch_thread_semaphore_create(void)
 {
 	semaphore_t s4;
 	kern_return_t kr;
 	while (unlikely(kr = semaphore_create(mach_task_self(), &s4,
 			SYNC_POLICY_FIFO, 0))) {
 		DISPATCH_VERIFY_MIG(kr);
 		_dispatch_temporary_resource_shortage();
 	}
 	return s4;
 }

 void
 _dispatch_thread_semaphore_dispose(void *ctxt)
 {
 	semaphore_t s4 = (semaphore_t)(uintptr_t)ctxt;
 	kern_return_t kr = semaphore_destroy(mach_task_self(), s4);
 	DISPATCH_VERIFY_MIG(kr);
 	DISPATCH_SEMAPHORE_VERIFY_KR(kr);
 }
 #endif

 void
 _dispatch_thread_event_signal_slow(dispatch_thread_event_t dte)
 {
 #if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
 	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
 		kern_return_t kr = semaphore_signal(dte->dte_semaphore);
 		DISPATCH_SEMAPHORE_VERIFY_KR(kr);
 		return;
 	}
 #endif
 #if HAVE_UL_COMPARE_AND_WAIT
 	_dispatch_ulock_wake(&dte->dte_value, 0);
 #elif HAVE_FUTEX
 	_dispatch_futex_wake(&dte->dte_value, 1, FUTEX_PRIVATE_FLAG);
 #elif USE_POSIX_SEM
 	int rc = sem_post(&dte->dte_sem);
 	DISPATCH_SEMAPHORE_VERIFY_RET(ret);
 #endif
 }

 void
 _dispatch_thread_event_wait_slow(dispatch_thread_event_t dte)
 {
 #if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
 	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
 		kern_return_t kr;
 		do {
 			kr = semaphore_wait(dte->dte_semaphore);
 		} while (unlikely(kr == KERN_ABORTED));
 		DISPATCH_SEMAPHORE_VERIFY_KR(kr);
 		return;
 	}
 #endif
 #if HAVE_UL_COMPARE_AND_WAIT || HAVE_FUTEX
 	for (;;) {
 		uint32_t value = os_atomic_load(&dte->dte_value, acquire);
 		if (likely(value == 0)) return;
 		if (unlikely(value != UINT32_MAX)) {
 			DISPATCH_CLIENT_CRASH(value, "Corrupt thread event value");
 		}
 #if HAVE_UL_COMPARE_AND_WAIT
 		int rc = _dispatch_ulock_wait(&dte->dte_value, UINT32_MAX, 0, 0);
 		dispatch_assert(rc == 0 || rc == EFAULT);
 #elif HAVE_FUTEX
 		_dispatch_futex_wait(&dte->dte_value, UINT32_MAX,
 				NULL, FUTEX_PRIVATE_FLAG);
 #endif
 	}
 #elif USE_POSIX_SEM
 	int rc;
 	do {
 		rc = sem_wait(&dte->dte_sem);
 	} while (unlikely(rc != 0));
 	DISPATCH_SEMAPHORE_VERIFY_RET(rc);
 #endif
 }

 #pragma mark - unfair lock

 #if HAVE_UL_UNFAIR_LOCK
 void
 _dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
 		dispatch_lock_options_t flags)
 {
 	dispatch_lock tid_self = _dispatch_tid_self(), next = tid_self;
 	dispatch_lock tid_old, tid_new;
 	int rc;

 	for (;;) {
 		os_atomic_rmw_loop(&dul->dul_lock, tid_old, tid_new, acquire, {
 			if (likely(!_dispatch_lock_is_locked(tid_old))) {
 				tid_new = next;
 			} else {
 				tid_new = tid_old & ~DLOCK_NOWAITERS_BIT;
 				if (tid_new == tid_old) os_atomic_rmw_loop_give_up(break);
 			}
 		});
 		if (unlikely(_dispatch_lock_is_locked_by(tid_old, tid_self))) {
 			DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
 		}
 		if (tid_new == next) {
 			return;
 		}
 		rc = _dispatch_unfair_lock_wait(&dul->dul_lock, tid_new, 0, flags);
 		if (rc == ENOTEMPTY) {
 			next = tid_self & ~DLOCK_NOWAITERS_BIT;
 		} else {
 			next = tid_self;
 		}
 	}
 }
 #elif HAVE_FUTEX
 void
 _dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
 		dispatch_lock_options_t flags)
 {
 	(void)flags;
 	_dispatch_futex_lock_pi(&dul->dul_lock, NULL, 1, FUTEX_PRIVATE_FLAG);
 }
 #else
 void
 _dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
 		dispatch_lock_options_t flags)
 {
 	dispatch_lock tid_cur, tid_self = _dispatch_tid_self();
 	uint32_t timeout = 1;

 	while (unlikely(!os_atomic_cmpxchgv(&dul->dul_lock,
 			DLOCK_OWNER_NULL, tid_self, &tid_cur, acquire))) {
 		if (unlikely(_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
 			DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
 		}
 		_dispatch_thread_switch(tid_cur, flags, timeout++);
 	}
 }
 #endif

 void
 _dispatch_unfair_lock_unlock_slow(dispatch_unfair_lock_t dul,
 		dispatch_lock tid_cur)
 {
 	dispatch_lock_owner tid_self = _dispatch_tid_self();
 	if (unlikely(!_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
 		DISPATCH_CLIENT_CRASH(tid_cur, "lock not owned by current thread");
 	}

 #if HAVE_UL_UNFAIR_LOCK
 	if (!(tid_cur & DLOCK_NOWAITERS_BIT)) {
 		_dispatch_unfair_lock_wake(&dul->dul_lock, 0);
 	}
 #elif HAVE_FUTEX
 	// futex_unlock_pi() handles both OWNER_DIED which we abuse & WAITERS
 	_dispatch_futex_unlock_pi(&dul->dul_lock, FUTEX_PRIVATE_FLAG);
 #else
 	(void)dul;
 #endif
 }

 #pragma mark - gate lock

 void
 _dispatch_gate_wait_slow(dispatch_gate_t dgl, dispatch_lock value,
 		dispatch_lock_options_t flags)
 {
 	dispatch_lock tid_self = _dispatch_tid_self(), tid_old, tid_new;
 	uint32_t timeout = 1;

 	for (;;) {
 		os_atomic_rmw_loop(&dgl->dgl_lock, tid_old, tid_new, acquire, {
 			if (likely(tid_old == value)) {
 				os_atomic_rmw_loop_give_up_with_fence(acquire, return);
 			}
 #ifdef DLOCK_NOWAITERS_BIT
 			tid_new = tid_old & ~DLOCK_NOWAITERS_BIT;
 #else
 			tid_new = tid_old | DLOCK_WAITERS_BIT;
 #endif
 			if (tid_new == tid_old) os_atomic_rmw_loop_give_up(break);
 		});
 		if (unlikely(_dispatch_lock_is_locked_by(tid_old, tid_self))) {
 			DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
 		}
 #if HAVE_UL_UNFAIR_LOCK
 		_dispatch_unfair_lock_wait(&dgl->dgl_lock, tid_new, 0, flags);
 #elif HAVE_FUTEX
 		_dispatch_futex_wait(&dgl->dgl_lock, tid_new, NULL, FUTEX_PRIVATE_FLAG);
 #else
 		_dispatch_thread_switch(tid_new, flags, timeout++);
 #endif
 		(void)timeout;
 	}
 }

 void
 _dispatch_gate_broadcast_slow(dispatch_gate_t dgl, dispatch_lock tid_cur)
 {
 	dispatch_lock_owner tid_self = _dispatch_tid_self();
 	if (unlikely(!_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
 		DISPATCH_CLIENT_CRASH(tid_cur, "lock not owned by current thread");
 	}

 #if HAVE_UL_UNFAIR_LOCK
 	_dispatch_unfair_lock_wake(&dgl->dgl_lock, ULF_WAKE_ALL);
 #elif HAVE_FUTEX
 	_dispatch_futex_wake(&dgl->dgl_lock, INT_MAX, FUTEX_PRIVATE_FLAG);
 #else
 	(void)dgl;
 #endif
 }
	/*
	* Copyright (c) 2016 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"

	#define _dlock_syscall_switch(err, syscall, ...) \
	for (;;) { \
	int err; \
	switch ((err = ((syscall) < 0 ? errno : 0))) { \
	case EINTR: continue; \
	__VA_ARGS__ \
	} \
	break; \
	}

	#if TARGET_OS_MAC
	_Static_assert(DLOCK_LOCK_DATA_CONTENTION == ULF_WAIT_WORKQ_DATA_CONTENTION,
	"values should be the same");

	DISPATCH_ALWAYS_INLINE
	static inline void
	_dispatch_thread_switch(dispatch_lock value, dispatch_lock_options_t flags,
	uint32_t timeout)
	{
	int option;
	if (flags & DLOCK_LOCK_DATA_CONTENTION) {
	option = SWITCH_OPTION_OSLOCK_DEPRESS;
	} else {
	option = SWITCH_OPTION_DEPRESS;
	}
	thread_switch(_dispatch_lock_owner(value), option, timeout);
	}
	#endif

	#pragma mark - ulock wrappers
	#if HAVE_UL_COMPARE_AND_WAIT

	static int
	_dispatch_ulock_wait(uint32_t *uaddr, uint32_t val, uint32_t timeout,
	uint32_t flags)
	{
	dispatch_assert(!DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK);
	int rc;
	_dlock_syscall_switch(err,
	rc = __ulock_wait(UL_COMPARE_AND_WAIT \| flags, uaddr, val, timeout),
	case 0: return rc > 0 ? ENOTEMPTY : 0;
	case ETIMEDOUT: case EFAULT: return err;
	default: DISPATCH_INTERNAL_CRASH(err, "ulock_wait() failed");
	);
	}

	static void
	_dispatch_ulock_wake(uint32_t *uaddr, uint32_t flags)
	{
	dispatch_assert(!DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK);
	_dlock_syscall_switch(err,
	__ulock_wake(UL_COMPARE_AND_WAIT \| flags, uaddr, 0),
	case 0: case ENOENT: break;
	default: DISPATCH_INTERNAL_CRASH(err, "ulock_wake() failed");
	);
	}

	#endif
	#if HAVE_UL_UNFAIR_LOCK

	// returns 0, ETIMEDOUT, ENOTEMPTY, EFAULT
	static int
	_dispatch_unfair_lock_wait(uint32_t *uaddr, uint32_t val, uint32_t timeout,
	dispatch_lock_options_t flags)
	{
	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
	// <rdar://problem/25075359>
	timeout = timeout < 1000 ? 1 : timeout / 1000;
	_dispatch_thread_switch(val, flags, timeout);
	return 0;
	}
	int rc;
	_dlock_syscall_switch(err,
	rc = __ulock_wait(UL_UNFAIR_LOCK \| flags, uaddr, val, timeout),
	case 0: return rc > 0 ? ENOTEMPTY : 0;
	case ETIMEDOUT: case EFAULT: return err;
	default: DISPATCH_INTERNAL_CRASH(err, "ulock_wait() failed");
	);
	}

	static void
	_dispatch_unfair_lock_wake(uint32_t *uaddr, uint32_t flags)
	{
	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
	// <rdar://problem/25075359>
	return;
	}
	_dlock_syscall_switch(err, __ulock_wake(UL_UNFAIR_LOCK \| flags, uaddr, 0),
	case 0: case ENOENT: break;
	default: DISPATCH_INTERNAL_CRASH(err, "ulock_wake() failed");
	);
	}

	#endif
	#pragma mark - futex wrappers
	#if HAVE_FUTEX
	#include <sys/time.h>
	#include <syscall.h>

	DISPATCH_ALWAYS_INLINE
	static inline int
	_dispatch_futex(uint32_t *uaddr, int op, uint32_t val,
	const struct timespec timeout, uint32_t uaddr2, uint32_t val3,
	int opflags)
	{
	return syscall(SYS_futex, uaddr, op \| opflags, val, timeout, uaddr2, val3);
	}

	static int
	_dispatch_futex_wait(uint32_t *uaddr, uint32_t val,
	const struct timespec *timeout, int opflags)
	{
	_dlock_syscall_switch(err,
	_dispatch_futex(uaddr, FUTEX_WAIT, val, timeout, NULL, 0, opflags),
	case 0: case EWOULDBLOCK: case ETIMEDOUT: return err;
	default: DISPATCH_CLIENT_CRASH(err, "futex_wait() failed");
	);
	}

	static void
	_dispatch_futex_wake(uint32_t *uaddr, int wake, int opflags)
	{
	int rc;
	_dlock_syscall_switch(err,
	rc = _dispatch_futex(uaddr, FUTEX_WAKE, wake, NULL, NULL, 0, opflags),
	case 0: return;
	default: DISPATCH_CLIENT_CRASH(err, "futex_wake() failed");
	);
	}

	static void
	_dispatch_futex_lock_pi(uint32_t uaddr, struct timespec timeout, int detect,
	int opflags)
	{
	_dlock_syscall_switch(err,
	_dispatch_futex(uaddr, FUTEX_LOCK_PI, detect, timeout,
	NULL, 0, opflags),
	case 0: return;
	default: DISPATCH_CLIENT_CRASH(errno, "futex_lock_pi() failed");
	);
	}

	static void
	_dispatch_futex_unlock_pi(uint32_t *uaddr, int opflags)
	{
	_dlock_syscall_switch(err,
	_dispatch_futex(uaddr, FUTEX_UNLOCK_PI, 0, NULL, NULL, 0, opflags),
	case 0: return;
	default: DISPATCH_CLIENT_CRASH(errno, "futex_unlock_pi() failed");
	);
	}

	#endif
	#pragma mark - wait for address

	void
	_dispatch_wait_on_address(uint32_t volatile *address, uint32_t value,
	dispatch_lock_options_t flags)
	{
	#if HAVE_UL_COMPARE_AND_WAIT
	_dispatch_ulock_wait((uint32_t *)address, value, 0, flags);
	#elif HAVE_FUTEX
	_dispatch_futex_wait((uint32_t *)address, value, NULL, FUTEX_PRIVATE_FLAG);
	#else
	mach_msg_timeout_t timeout = 1;
	while (os_atomic_load(address, relaxed) == value) {
	thread_switch(MACH_PORT_NULL, SWITCH_OPTION_WAIT, timeout++);
	}
	#endif
	(void)flags;
	}

	void
	_dispatch_wake_by_address(uint32_t volatile *address)
	{
	#if HAVE_UL_COMPARE_AND_WAIT
	_dispatch_ulock_wake((uint32_t *)address, ULF_WAKE_ALL);
	#elif HAVE_FUTEX
	_dispatch_futex_wake((uint32_t *)address, INT_MAX, FUTEX_PRIVATE_FLAG);
	#else
	(void)address;
	#endif
	}

	#pragma mark - thread event

	#if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
	semaphore_t
	_dispatch_thread_semaphore_create(void)
	{
	semaphore_t s4;
	kern_return_t kr;
	while (unlikely(kr = semaphore_create(mach_task_self(), &s4,
	SYNC_POLICY_FIFO, 0))) {
	DISPATCH_VERIFY_MIG(kr);
	_dispatch_temporary_resource_shortage();
	}
	return s4;
	}

	void
	_dispatch_thread_semaphore_dispose(void *ctxt)
	{
	semaphore_t s4 = (semaphore_t)(uintptr_t)ctxt;
	kern_return_t kr = semaphore_destroy(mach_task_self(), s4);
	DISPATCH_VERIFY_MIG(kr);
	DISPATCH_SEMAPHORE_VERIFY_KR(kr);
	}
	#endif

	void
	_dispatch_thread_event_signal_slow(dispatch_thread_event_t dte)
	{
	#if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
	kern_return_t kr = semaphore_signal(dte->dte_semaphore);
	DISPATCH_SEMAPHORE_VERIFY_KR(kr);
	return;
	}
	#endif
	#if HAVE_UL_COMPARE_AND_WAIT
	_dispatch_ulock_wake(&dte->dte_value, 0);
	#elif HAVE_FUTEX
	_dispatch_futex_wake(&dte->dte_value, 1, FUTEX_PRIVATE_FLAG);
	#elif USE_POSIX_SEM
	int rc = sem_post(&dte->dte_sem);
	DISPATCH_SEMAPHORE_VERIFY_RET(ret);
	#endif
	}

	void
	_dispatch_thread_event_wait_slow(dispatch_thread_event_t dte)
	{
	#if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
	if (DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK) {
	kern_return_t kr;
	do {
	kr = semaphore_wait(dte->dte_semaphore);
	} while (unlikely(kr == KERN_ABORTED));
	DISPATCH_SEMAPHORE_VERIFY_KR(kr);
	return;
	}
	#endif
	#if HAVE_UL_COMPARE_AND_WAIT \|\| HAVE_FUTEX
	for (;;) {
	uint32_t value = os_atomic_load(&dte->dte_value, acquire);
	if (likely(value == 0)) return;
	if (unlikely(value != UINT32_MAX)) {
	DISPATCH_CLIENT_CRASH(value, "Corrupt thread event value");
	}
	#if HAVE_UL_COMPARE_AND_WAIT
	int rc = _dispatch_ulock_wait(&dte->dte_value, UINT32_MAX, 0, 0);
	dispatch_assert(rc == 0 \|\| rc == EFAULT);
	#elif HAVE_FUTEX
	_dispatch_futex_wait(&dte->dte_value, UINT32_MAX,
	NULL, FUTEX_PRIVATE_FLAG);
	#endif
	}
	#elif USE_POSIX_SEM
	int rc;
	do {
	rc = sem_wait(&dte->dte_sem);
	} while (unlikely(rc != 0));
	DISPATCH_SEMAPHORE_VERIFY_RET(rc);
	#endif
	}

	#pragma mark - unfair lock

	#if HAVE_UL_UNFAIR_LOCK
	void
	_dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
	dispatch_lock_options_t flags)
	{
	dispatch_lock tid_self = _dispatch_tid_self(), next = tid_self;
	dispatch_lock tid_old, tid_new;
	int rc;

	for (;;) {
	os_atomic_rmw_loop(&dul->dul_lock, tid_old, tid_new, acquire, {
	if (likely(!_dispatch_lock_is_locked(tid_old))) {
	tid_new = next;
	} else {
	tid_new = tid_old & ~DLOCK_NOWAITERS_BIT;
	if (tid_new == tid_old) os_atomic_rmw_loop_give_up(break);
	}
	});
	if (unlikely(_dispatch_lock_is_locked_by(tid_old, tid_self))) {
	DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
	}
	if (tid_new == next) {
	return;
	}
	rc = _dispatch_unfair_lock_wait(&dul->dul_lock, tid_new, 0, flags);
	if (rc == ENOTEMPTY) {
	next = tid_self & ~DLOCK_NOWAITERS_BIT;
	} else {
	next = tid_self;
	}
	}
	}
	#elif HAVE_FUTEX
	void
	_dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
	dispatch_lock_options_t flags)
	{
	(void)flags;
	_dispatch_futex_lock_pi(&dul->dul_lock, NULL, 1, FUTEX_PRIVATE_FLAG);
	}
	#else
	void
	_dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
	dispatch_lock_options_t flags)
	{
	dispatch_lock tid_cur, tid_self = _dispatch_tid_self();
	uint32_t timeout = 1;

	while (unlikely(!os_atomic_cmpxchgv(&dul->dul_lock,
	DLOCK_OWNER_NULL, tid_self, &tid_cur, acquire))) {
	if (unlikely(_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
	DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
	}
	_dispatch_thread_switch(tid_cur, flags, timeout++);
	}
	}
	#endif

	void
	_dispatch_unfair_lock_unlock_slow(dispatch_unfair_lock_t dul,
	dispatch_lock tid_cur)
	{
	dispatch_lock_owner tid_self = _dispatch_tid_self();
	if (unlikely(!_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
	DISPATCH_CLIENT_CRASH(tid_cur, "lock not owned by current thread");
	}

	#if HAVE_UL_UNFAIR_LOCK
	if (!(tid_cur & DLOCK_NOWAITERS_BIT)) {
	_dispatch_unfair_lock_wake(&dul->dul_lock, 0);
	}
	#elif HAVE_FUTEX
	// futex_unlock_pi() handles both OWNER_DIED which we abuse & WAITERS
	_dispatch_futex_unlock_pi(&dul->dul_lock, FUTEX_PRIVATE_FLAG);
	#else
	(void)dul;
	#endif
	}

	#pragma mark - gate lock

	void
	_dispatch_gate_wait_slow(dispatch_gate_t dgl, dispatch_lock value,
	dispatch_lock_options_t flags)
	{
	dispatch_lock tid_self = _dispatch_tid_self(), tid_old, tid_new;
	uint32_t timeout = 1;

	for (;;) {
	os_atomic_rmw_loop(&dgl->dgl_lock, tid_old, tid_new, acquire, {
	if (likely(tid_old == value)) {
	os_atomic_rmw_loop_give_up_with_fence(acquire, return);
	}
	#ifdef DLOCK_NOWAITERS_BIT
	tid_new = tid_old & ~DLOCK_NOWAITERS_BIT;
	#else
	tid_new = tid_old \| DLOCK_WAITERS_BIT;
	#endif
	if (tid_new == tid_old) os_atomic_rmw_loop_give_up(break);
	});
	if (unlikely(_dispatch_lock_is_locked_by(tid_old, tid_self))) {
	DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
	}
	#if HAVE_UL_UNFAIR_LOCK
	_dispatch_unfair_lock_wait(&dgl->dgl_lock, tid_new, 0, flags);
	#elif HAVE_FUTEX
	_dispatch_futex_wait(&dgl->dgl_lock, tid_new, NULL, FUTEX_PRIVATE_FLAG);
	#else
	_dispatch_thread_switch(tid_new, flags, timeout++);
	#endif
	(void)timeout;
	}
	}

	void
	_dispatch_gate_broadcast_slow(dispatch_gate_t dgl, dispatch_lock tid_cur)
	{
	dispatch_lock_owner tid_self = _dispatch_tid_self();
	if (unlikely(!_dispatch_lock_is_locked_by(tid_cur, tid_self))) {
	DISPATCH_CLIENT_CRASH(tid_cur, "lock not owned by current thread");
	}

	#if HAVE_UL_UNFAIR_LOCK
	_dispatch_unfair_lock_wake(&dgl->dgl_lock, ULF_WAKE_ALL);
	#elif HAVE_FUTEX
	_dispatch_futex_wake(&dgl->dgl_lock, INT_MAX, FUTEX_PRIVATE_FLAG);
	#else
	(void)dgl;
	#endif
	}