semaphore.c

/*
 * Copyright (c) 2008-2013 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"

DISPATCH_WEAK // rdar://problem/8503746
intptr_t _dispatch_semaphore_signal_slow(dispatch_semaphore_t dsema);

#pragma mark -
#pragma mark dispatch_semaphore_t

dispatch_semaphore_t
dispatch_semaphore_create(intptr_t 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 DISPATCH_BAD_INPUT;
	}

	dsema = _dispatch_object_alloc(DISPATCH_VTABLE(semaphore),
			sizeof(struct dispatch_semaphore_s));
	dsema->do_next = DISPATCH_OBJECT_LISTLESS;
	dsema->do_targetq = _dispatch_get_default_queue(false);
	dsema->dsema_value = value;
	_dispatch_sema4_init(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
	dsema->dsema_orig = value;
	return dsema;
}

void
_dispatch_semaphore_dispose(dispatch_object_t dou,
		DISPATCH_UNUSED bool *allow_free)
{
	dispatch_semaphore_t dsema = dou._dsema;

	if (dsema->dsema_value < dsema->dsema_orig) {
		DISPATCH_CLIENT_CRASH(dsema->dsema_orig - dsema->dsema_value,
				"Semaphore object deallocated while in use");
	}

	_dispatch_sema4_dispose(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
}

size_t
_dispatch_semaphore_debug(dispatch_object_t dou, char *buf, size_t bufsiz)
{
	dispatch_semaphore_t dsema = dou._dsema;

	size_t offset = 0;
	offset += dsnprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ",
			_dispatch_object_class_name(dsema), dsema);
	offset += _dispatch_object_debug_attr(dsema, &buf[offset], bufsiz - offset);
#if USE_MACH_SEM
	offset += dsnprintf(&buf[offset], bufsiz - offset, "port = 0x%u, ",
			dsema->dsema_sema);
#endif
	offset += dsnprintf(&buf[offset], bufsiz - offset,
			"value = %" PRIdPTR ", orig = %" PRIdPTR " }", dsema->dsema_value, dsema->dsema_orig);
	return offset;
}

DISPATCH_NOINLINE
intptr_t
_dispatch_semaphore_signal_slow(dispatch_semaphore_t dsema)
{
	_dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
	_dispatch_sema4_signal(&dsema->dsema_sema, 1);
	return 1;
}

intptr_t
dispatch_semaphore_signal(dispatch_semaphore_t dsema)
{
	long value = os_atomic_inc2o(dsema, dsema_value, release);
	if (likely(value > 0)) {
		return 0;
	}
	if (unlikely(value == LONG_MIN)) {
		DISPATCH_CLIENT_CRASH(value,
				"Unbalanced call to dispatch_semaphore_signal()");
	}
	return _dispatch_semaphore_signal_slow(dsema);
}

DISPATCH_NOINLINE
static intptr_t
_dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema,
		dispatch_time_t timeout)
{
	long orig;

	_dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
	switch (timeout) {
	default:
		if (!_dispatch_sema4_timedwait(&dsema->dsema_sema, timeout)) {
			break;
		}
		// Try to undo what the fast path did to dsema->dsema_value
		DISPATCH_FALLTHROUGH;
	case DISPATCH_TIME_NOW:
		orig = dsema->dsema_value;
		while (orig < 0) {
			if (os_atomic_cmpxchgvw2o(dsema, dsema_value, orig, orig + 1,
					&orig, relaxed)) {
				return _DSEMA4_TIMEOUT();
			}
		}
		// Another thread called semaphore_signal(). Drain the wakeup.
		DISPATCH_FALLTHROUGH;
	case DISPATCH_TIME_FOREVER:
		_dispatch_sema4_wait(&dsema->dsema_sema);
		break;
	}
	return 0;
}

intptr_t
dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
	long value = os_atomic_dec2o(dsema, dsema_value, acquire);
	if (likely(value >= 0)) {
		return 0;
	}
	return _dispatch_semaphore_wait_slow(dsema, timeout);
}

#pragma mark -
#pragma mark dispatch_group_t

DISPATCH_ALWAYS_INLINE
static inline dispatch_group_t
_dispatch_group_create_with_count(uint32_t n)
{
	dispatch_group_t dg = _dispatch_object_alloc(DISPATCH_VTABLE(group),
			sizeof(struct dispatch_group_s));
	dg->do_next = DISPATCH_OBJECT_LISTLESS;
	dg->do_targetq = _dispatch_get_default_queue(false);
	if (n) {
		os_atomic_store2o(dg, dg_bits,
				(uint32_t)-n * DISPATCH_GROUP_VALUE_INTERVAL, relaxed);
		os_atomic_store2o(dg, do_ref_cnt, 1, relaxed); // <rdar://22318411>
	}
	return dg;
}

dispatch_group_t
dispatch_group_create(void)
{
	return _dispatch_group_create_with_count(0);
}

dispatch_group_t
_dispatch_group_create_and_enter(void)
{
	return _dispatch_group_create_with_count(1);
}

void
_dispatch_group_dispose(dispatch_object_t dou, DISPATCH_UNUSED bool *allow_free)
{
	uint64_t dg_state = os_atomic_load2o(dou._dg, dg_state, relaxed);

	if (unlikely((uint32_t)dg_state)) {
		DISPATCH_CLIENT_CRASH((uintptr_t)dg_state,
				"Group object deallocated while in use");
	}
}

size_t
_dispatch_group_debug(dispatch_object_t dou, char *buf, size_t bufsiz)
{
	dispatch_group_t dg = dou._dg;
	uint64_t dg_state = os_atomic_load2o(dg, dg_state, relaxed);

	size_t offset = 0;
	offset += dsnprintf(&buf[offset], bufsiz - offset, "%s[%p] = { ",
			_dispatch_object_class_name(dg), dg);
	offset += _dispatch_object_debug_attr(dg, &buf[offset], bufsiz - offset);
	offset += dsnprintf(&buf[offset], bufsiz - offset,
			"count = %u, gen = %d, waiters = %d, notifs = %d }",
			_dg_state_value(dg_state), _dg_state_gen(dg_state),
			(bool)(dg_state & DISPATCH_GROUP_HAS_WAITERS),
			(bool)(dg_state & DISPATCH_GROUP_HAS_NOTIFS));
	return offset;
}

DISPATCH_NOINLINE
static intptr_t
_dispatch_group_wait_slow(dispatch_group_t dg, uint32_t gen,
		dispatch_time_t timeout)
{
	for (;;) {
		int rc = _dispatch_wait_on_address(&dg->dg_gen, gen, timeout, 0);
		if (likely(gen != os_atomic_load2o(dg, dg_gen, acquire))) {
			return 0;
		}
		if (rc == ETIMEDOUT) {
			return _DSEMA4_TIMEOUT();
		}
	}
}

intptr_t
dispatch_group_wait(dispatch_group_t dg, dispatch_time_t timeout)
{
	uint64_t old_state, new_state;

	os_atomic_rmw_loop2o(dg, dg_state, old_state, new_state, relaxed, {
		if ((old_state & DISPATCH_GROUP_VALUE_MASK) == 0) {
			os_atomic_rmw_loop_give_up_with_fence(acquire, return 0);
		}
		if (unlikely(timeout == 0)) {
			os_atomic_rmw_loop_give_up(return _DSEMA4_TIMEOUT());
		}
		new_state = old_state | DISPATCH_GROUP_HAS_WAITERS;
		if (unlikely(old_state & DISPATCH_GROUP_HAS_WAITERS)) {
			os_atomic_rmw_loop_give_up(break);
		}
	});

	return _dispatch_group_wait_slow(dg, _dg_state_gen(new_state), timeout);
}

DISPATCH_NOINLINE
static void
_dispatch_group_wake(dispatch_group_t dg, uint64_t dg_state, bool needs_release)
{
	uint16_t refs = needs_release ? 1 : 0; // <rdar://problem/22318411>

	if (dg_state & DISPATCH_GROUP_HAS_NOTIFS) {
		dispatch_continuation_t dc, next_dc, tail;

		// Snapshot before anything is notified/woken <rdar://problem/8554546>
		dc = os_mpsc_capture_snapshot(os_mpsc(dg, dg_notify), &tail);
		do {
			dispatch_queue_t dsn_queue = (dispatch_queue_t)dc->dc_data;
			next_dc = os_mpsc_pop_snapshot_head(dc, tail, do_next);
			_dispatch_continuation_async(dsn_queue, dc,
					_dispatch_qos_from_pp(dc->dc_priority), dc->dc_flags);
			_dispatch_release(dsn_queue);
		} while ((dc = next_dc));

		refs++;
	}

	if (dg_state & DISPATCH_GROUP_HAS_WAITERS) {
		_dispatch_wake_by_address(&dg->dg_gen);
	}

	if (refs) _dispatch_release_n(dg, refs);
}

void
dispatch_group_leave(dispatch_group_t dg)
{
	// The value is incremented on a 64bits wide atomic so that the carry for
	// the -1 -> 0 transition increments the generation atomically.
	uint64_t new_state, old_state = os_atomic_add_orig2o(dg, dg_state,
			DISPATCH_GROUP_VALUE_INTERVAL, release);
	uint32_t old_value = (uint32_t)(old_state & DISPATCH_GROUP_VALUE_MASK);

	if (unlikely(old_value == DISPATCH_GROUP_VALUE_1)) {
		old_state += DISPATCH_GROUP_VALUE_INTERVAL;
		do {
			new_state = old_state;
			if ((old_state & DISPATCH_GROUP_VALUE_MASK) == 0) {
				new_state &= ~DISPATCH_GROUP_HAS_WAITERS;
				new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
			} else {
				// If the group was entered again since the atomic_add above,
				// we can't clear the waiters bit anymore as we don't know for
				// which generation the waiters are for
				new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
			}
			if (old_state == new_state) break;
		} while (unlikely(!os_atomic_cmpxchgv2o(dg, dg_state,
				old_state, new_state, &old_state, relaxed)));
		return _dispatch_group_wake(dg, old_state, true);
	}

	if (unlikely(old_value == 0)) {
		DISPATCH_CLIENT_CRASH((uintptr_t)old_value,
				"Unbalanced call to dispatch_group_leave()");
	}
}

void
dispatch_group_enter(dispatch_group_t dg)
{
	// The value is decremented on a 32bits wide atomic so that the carry
	// for the 0 -> -1 transition is not propagated to the upper 32bits.
	uint32_t old_bits = os_atomic_sub_orig2o(dg, dg_bits,
			DISPATCH_GROUP_VALUE_INTERVAL, acquire);
	uint32_t old_value = old_bits & DISPATCH_GROUP_VALUE_MASK;
	if (unlikely(old_value == 0)) {
		_dispatch_retain(dg); // <rdar://problem/22318411>
	}
	if (unlikely(old_value == DISPATCH_GROUP_VALUE_MAX)) {
		DISPATCH_CLIENT_CRASH(old_bits,
				"Too many nested calls to dispatch_group_enter()");
	}
}

DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_continuation_t dsn)
{
	uint64_t old_state, new_state;
	dispatch_continuation_t prev;

	dsn->dc_data = dq;
	_dispatch_retain(dq);

	prev = os_mpsc_push_update_tail(os_mpsc(dg, dg_notify), dsn, do_next);
	if (os_mpsc_push_was_empty(prev)) _dispatch_retain(dg);
	os_mpsc_push_update_prev(os_mpsc(dg, dg_notify), prev, dsn, do_next);
	if (os_mpsc_push_was_empty(prev)) {
		os_atomic_rmw_loop2o(dg, dg_state, old_state, new_state, release, {
			new_state = old_state | DISPATCH_GROUP_HAS_NOTIFS;
			if ((uint32_t)old_state == 0) {
				os_atomic_rmw_loop_give_up({
					return _dispatch_group_wake(dg, new_state, false);
				});
			}
		});
	}
}

DISPATCH_NOINLINE
void
dispatch_group_notify_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func)
{
	dispatch_continuation_t dsn = _dispatch_continuation_alloc();
	_dispatch_continuation_init_f(dsn, dq, ctxt, func, 0, DC_FLAG_CONSUME);
	_dispatch_group_notify(dg, dq, dsn);
}

#ifdef __BLOCKS__
void
dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_block_t db)
{
	dispatch_continuation_t dsn = _dispatch_continuation_alloc();
	_dispatch_continuation_init(dsn, dq, db, 0, DC_FLAG_CONSUME);
	_dispatch_group_notify(dg, dq, dsn);
}
#endif

DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_continuation_group_async(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_continuation_t dc, dispatch_qos_t qos)
{
	dispatch_group_enter(dg);
	dc->dc_data = dg;
	_dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}

DISPATCH_NOINLINE
void
dispatch_group_async_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func)
{
	dispatch_continuation_t dc = _dispatch_continuation_alloc();
	uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_GROUP_ASYNC;
	dispatch_qos_t qos;

	qos = _dispatch_continuation_init_f(dc, dq, ctxt, func, 0, dc_flags);
	_dispatch_continuation_group_async(dg, dq, dc, qos);
}

#ifdef __BLOCKS__
void
dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_block_t db)
{
	dispatch_continuation_t dc = _dispatch_continuation_alloc();
	uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_GROUP_ASYNC;
	dispatch_qos_t qos;

	qos = _dispatch_continuation_init(dc, dq, db, 0, dc_flags);
	_dispatch_continuation_group_async(dg, dq, dc, qos);
}
#endif