• [OC学习笔记]多线程之GCD

    串行队列和并发队列的源码解析

    在我们的开发过程中，使用队列的时候，苹果给我们给了3个获取队列的方法：

    //主队列
dispatch_queue_t mainQueue = dispatch_get_main_queue();
//全局并发队列
dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
//自己创建的队列
dispatch_queue_t normalQueue = dispatch_queue_create("com.test.serial", DISPATCH_QUEUE_SERIAL);

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    dispatch_get_main_queue

    打开源码，可以在queue.h里面找到对应的代码：

    dispatch_queue_main_t
dispatch_get_main_queue(void)
{
	return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
}

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    接下来找DISPATCH_GLOBAL_OBJECT：

    #define DISPATCH_GLOBAL_OBJECT(type, object) ((type)&(object))
// dispatch_queue_main_t & _dispatch_main_q 

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    可以得出类型是dispatch_queue_main_t，对象是_dispatch_main_q，继续搜索_dispatch_main_q：

    struct dispatch_queue_static_s _dispatch_main_q = {
	DISPATCH_GLOBAL_OBJECT_HEADER(queue_main),
#if !DISPATCH_USE_RESOLVERS
	.do_targetq = _dispatch_get_default_queue(true),
#endif
	.dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(1) |
			DISPATCH_QUEUE_ROLE_BASE_ANON,
	.dq_label = "com.apple.main-thread",
	.dq_atomic_flags = DQF_THREAD_BOUND | DQF_WIDTH(1),
	.dq_serialnum = 1,
};

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    可以看到：主队列的lable = com.apple.main-thread。dq_serialnum = 1就说明是一个串行队列，那么队列是怎么创建的呢？我们知道这里用到了一个函数dispatch_queue_create，接下来就先探索一下吧。

    dispatch_queue_create

    我们再来看一看如何创建队列，打开源码，找到dispatch_queue_create：

    // queue.c
dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
	return _dispatch_lane_create_with_target(label, attr,
			DISPATCH_TARGET_QUEUE_DEFAULT, true);
}

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    _dispatch_lane_create_with_target

    可以看到是调用_dispatch_lane_create_with_target并添加了2个默认参数实现的，找到它的对应实现：

    static dispatch_queue_t
_dispatch_lane_create_with_target(const char *label, dispatch_queue_attr_t dqa,
		dispatch_queue_t tq, bool legacy)
{
	dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);

	// 优先级的处理
	// Step 1: Normalize arguments (qos, overcommit, tq)
	//

	...

	// 初始化queue
	// Step 2: Initialize the queue
	//

	...
	
	// 申请和开辟内存
	dispatch_lane_t dq = _dispatch_object_alloc(vtable,
			sizeof(struct dispatch_lane_s));
	// 构造函数初始化  dqai.dqai_concurrent：是否是并发
	_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
			DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
			(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));

	dq->dq_label = label;
	dq->dq_priority = _dispatch_priority_make((dispatch_qos_t)dqai.dqai_qos,
			dqai.dqai_relpri);
	if (overcommit == _dispatch_queue_attr_overcommit_enabled) {
		dq->dq_priority |= DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
	}
	if (!dqai.dqai_inactive) {
		_dispatch_queue_priority_inherit_from_target(dq, tq);
		_dispatch_lane_inherit_wlh_from_target(dq, tq);
	}
	_dispatch_retain(tq);
	dq->do_targetq = tq;
	_dispatch_object_debug(dq, "%s", __func__);
	return _dispatch_trace_queue_create(dq)._dq;
}

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    第一行中的_dispatch_queue_attr_to_info方法里面，把我们传入的DISPATCH_QUEUE_SERIAL或者DISPATCH_QUEUE_CONCURRENT参数进行封装，封装成了dqai。我们可以大致看看封装的实现：

    dispatch_queue_attr_info_t
_dispatch_queue_attr_to_info(dispatch_queue_attr_t dqa)
{
	dispatch_queue_attr_info_t dqai = { };
	// 串行队列直接返回空的 dqai 结构体
	if (!dqa) return dqai;

#if DISPATCH_VARIANT_STATIC
	if (dqa == &_dispatch_queue_attr_concurrent) {
		// ⚠️
		dqai.dqai_concurrent = true;
		return dqai;
	}
#endif

	if (dqa < _dispatch_queue_attrs ||
			dqa >= &_dispatch_queue_attrs[DISPATCH_QUEUE_ATTR_COUNT]) {
#ifndef __APPLE__
		if (memcmp(dqa, &_dispatch_queue_attrs[0],
				sizeof(struct dispatch_queue_attr_s)) == 0) {
			dqa = (dispatch_queue_attr_t)&_dispatch_queue_attrs[0];
		} else
#endif // __APPLE__
		DISPATCH_CLIENT_CRASH(dqa->do_vtable, "Invalid queue attribute");
	}

	size_t idx = (size_t)(dqa - _dispatch_queue_attrs);
	
	// 并发队列结构体位域的默认配置和赋值
	dqai.dqai_inactive = (idx % DISPATCH_QUEUE_ATTR_INACTIVE_COUNT);
	idx /= DISPATCH_QUEUE_ATTR_INACTIVE_COUNT;
	// ⚠️⚠️
	dqai.dqai_concurrent = !(idx % DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT);
	idx /= DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT;

	dqai.dqai_relpri = -(int)(idx % DISPATCH_QUEUE_ATTR_PRIO_COUNT);
	idx /= DISPATCH_QUEUE_ATTR_PRIO_COUNT;

	dqai.dqai_qos = idx % DISPATCH_QUEUE_ATTR_QOS_COUNT;
	idx /= DISPATCH_QUEUE_ATTR_QOS_COUNT;

	dqai.dqai_autorelease_frequency =
			idx % DISPATCH_QUEUE_ATTR_AUTORELEASE_FREQUENCY_COUNT;
	idx /= DISPATCH_QUEUE_ATTR_AUTORELEASE_FREQUENCY_COUNT;

	dqai.dqai_overcommit = idx % DISPATCH_QUEUE_ATTR_OVERCOMMIT_COUNT;
	idx /= DISPATCH_QUEUE_ATTR_OVERCOMMIT_COUNT;

	return dqai;
}

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    dqai里面有个dqai_concurrent的属性，顾名思义是代表是否是并发，那么默认的就是串行。
    接下来继续看如何根据dqai创建队列的：

    // 构造函数初始化  dqai.dqai_concurrent：是否是并发
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
		DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
		(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));

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    可以看到通过init方法初始化，第三个参数，如果是并发传入DISPATCH_QUEUE_WIDTH_MAX，如果是串行传入1。

    #define DISPATCH_QUEUE_WIDTH_FULL_BIT		0x0020000000000000ull
#define DISPATCH_QUEUE_WIDTH_FULL			0x1000ull
#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1)
#define DISPATCH_QUEUE_WIDTH_MAX  (DISPATCH_QUEUE_WIDTH_FULL - 2)
#define DISPATCH_QUEUE_USES_REDIRECTION(width) \
		({ uint16_t _width = (width); \
		_width > 1 && _width < DISPATCH_QUEUE_WIDTH_POOL; })

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    而这里是DISPATCH_QUEUE_WIDTH_MAX的定义，可以计算其结果是14。

    _dispatch_queue_init

    我们再看init函数内部实现：

    static inline dispatch_queue_class_t
_dispatch_queue_init(dispatch_queue_class_t dqu, dispatch_queue_flags_t dqf,
		uint16_t width, uint64_t initial_state_bits)
{
	uint64_t dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(width);
	dispatch_queue_t dq = dqu._dq;

	dispatch_assert((initial_state_bits & ~(DISPATCH_QUEUE_ROLE_MASK |
			DISPATCH_QUEUE_INACTIVE)) == 0);

	if (initial_state_bits & DISPATCH_QUEUE_INACTIVE) {
		dq->do_ref_cnt += 2; // rdar://8181908 see _dispatch_lane_resume
		if (dx_metatype(dq) == _DISPATCH_SOURCE_TYPE) {
			dq->do_ref_cnt++; // released when DSF_DELETED is set
		}
	}

	dq_state |= initial_state_bits;
	dq->do_next = DISPATCH_OBJECT_LISTLESS;
	// ⚠️⚠️
	dqf |= DQF_WIDTH(width);
	os_atomic_store2o(dq, dq_atomic_flags, dqf, relaxed);
	dq->dq_state = dq_state;
	dq->dq_serialnum =
			os_atomic_inc_orig(&_dispatch_queue_serial_numbers, relaxed);
	return dqu;
}

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    可以得出：
    如果是并发队列dqf |= DQF_WIDTH(DISPATCH_QUEUE_WIDTH_MAX) 如果是串行队列dqf |= DQF_WIDTH(1)。串行队列和并发队列最根本的区别就是DQF_WIDTH不同，串行队列的为1。这个width可以抽象的理解为队列出口的宽度。可以把串行队列想成一个单向单车道，把任务想成一辆辆车子，车子通过的时候必须一辆一辆按顺序通过；而并发队列可以想成单向多车道，有多个出口，车子可以并行通过。
    继续看dq->dq_serialnum = os_atomic_inc_orig(&_dispatch_queue_serial_numbers, relaxed);：

    // skip zero
// 1 - main_q
// 2 - mgr_q
// 3 - mgr_root_q
// 4,5,6,7,8,9,10,11,12,13,14,15 - global queues
// 17 - workloop_fallback_q
// we use 'xadd' on Intel, so the initial value == next assigned
#define DISPATCH_QUEUE_SERIAL_NUMBER_INIT 17
extern unsigned long volatile _dispatch_queue_serial_numbers;

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    所以这里的_dispatch_queue_serial_numbers只是代表的是创建的队列的归属（串行还是并发），所以上面的dq->dq_serialnum = 1就是创建的主队列也是串行队列 。
    再回到_dispatch_lane_create_with_target，看到下面有：

    _dispatch_retain(tq);
dq->do_targetq = tq;

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    这个 tq 是在哪赋值的呢？向上找，在省略的部分找到：

    // priority.h
#define DISPATCH_QOS_UNSPECIFIED        ((dispatch_qos_t)0)
#define DISPATCH_QOS_DEFAULT            ((dispatch_qos_t)4)

// queue.c
    ...
	else {
		if (overcommit == _dispatch_queue_attr_overcommit_unspecified) {
			// Serial queues default to overcommit!
			// 如果是并发 overcommit = _dispatch_queue_attr_overcommit_disabled
			// 如果是串行 overcommit = _dispatch_queue_attr_overcommit_enabled
			overcommit = dqai.dqai_concurrent ?
					_dispatch_queue_attr_overcommit_disabled :
					_dispatch_queue_attr_overcommit_enabled;
		}
	}
	if (!tq) {
		tq = _dispatch_get_root_queue(
				qos == DISPATCH_QOS_UNSPECIFIED ? DISPATCH_QOS_DEFAULT : qos,
				overcommit == _dispatch_queue_attr_overcommit_enabled)->_as_dq;
		if (unlikely(!tq)) {
			DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");
		}
	}
	...

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    static inline dispatch_queue_global_t
_dispatch_get_root_queue(dispatch_qos_t qos, bool overcommit)
{
	if (unlikely(qos < DISPATCH_QOS_MIN || qos > DISPATCH_QOS_MAX)) {
		DISPATCH_CLIENT_CRASH(qos, "Corrupted priority");
	}
	// qos 为 4，4-1= 3
	// 2*3 + 0或者1 = 6/7
	// 然后再去数组 _dispatch_root_queues 里取数组的 6 或者 7 的下标指针地址
	return &_dispatch_root_queues[2 * (qos - 1) + overcommit];
}

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    tq 的值是通过 _dispatch_root_queues 数组取出来的，直接到数组里面看就一目了然了。由此可以发现 tq 就是 dq_label 的值，也就是外面队列 target 的值。

    // 串行队列
_DISPATCH_ROOT_QUEUE_ENTRY(UTILITY, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
	.dq_label = "com.apple.root.utility-qos.overcommit",
	.dq_serialnum = 9,
),
// 并发队列(全局和并发是一样的)
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT, DISPATCH_PRIORITY_FLAG_FALLBACK,
	.dq_label = "com.apple.root.default-qos",
	.dq_serialnum = 10,
),
// 主队列
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT,
		DISPATCH_PRIORITY_FLAG_FALLBACK | DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
	.dq_label = "com.apple.root.default-qos.overcommit",
	.dq_serialnum = 11,
),

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    既然串行队列和并发队列的 target 信息是从 _dispatch_root_queues 结构体数组取出来的，那么 _dispatch_root_queues 又是在哪创建的呢？我们来到最先初始化的 libdispatcdispatch_queue_createh_init 里的查找，最终在 _dispatch_introspection_init 里找到一些代码：
    请添加图片描述
    队列是通过 for 循环，调用 _dispatch_trace_queue_create，再取出 _dispatch_root_queues 里的地址指针一个一个创建出来的。

    GCD深入了解

    执行任务的方式

    执行任务的函数分为两种，同步和异步函数：

    • 同步函数：即 dispatch_sync，必须等待当前语句执行完毕，才会执行下一条语
      句，不会开启线程，在当前执行任务。
    • 异步函数，即 dispatch_async。不用等待当前语句执行完毕，就可以执行下一条语句，会开启线程执行任务。

    dispatch_sync 同步源码分析

    找到源码中同步函数的实现：

    void
dispatch_sync(dispatch_queue_t dq, dispatch_block_t work)
{
	uintptr_t dc_flags = DC_FLAG_BLOCK;
	if (unlikely(_dispatch_block_has_private_data(work))) {
		return _dispatch_sync_block_with_privdata(dq, work, dc_flags);
	}
	_dispatch_sync_f(dq, work, _dispatch_Block_invoke(work), dc_flags);
}

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    unlikely 的意思是基本上不会走，然后就来到 _dispatch_sync_f 函数，_dispatch_sync_f 的第三个参数是将 block 包装了一下：

    #define _dispatch_Block_invoke(bb) \
		((dispatch_function_t)((struct Block_layout *)bb)->invoke)

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    继续往下看：

    static void
_dispatch_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func,
		uintptr_t dc_flags)
{
	_dispatch_sync_f_inline(dq, ctxt, func, dc_flags);
}

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    之后就会来到 _dispatch_sync_f_inline 函数，实现如下：

    static inline void
_dispatch_sync_f_inline(dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func, uintptr_t dc_flags)
{
	// 串行就会走这下面
	if (likely(dq->dq_width == 1)) {
		return _dispatch_barrier_sync_f(dq, ctxt, func, dc_flags);
	}

	if (unlikely(dx_metatype(dq) != _DISPATCH_LANE_TYPE)) {
		DISPATCH_CLIENT_CRASH(0, "Queue type doesn't support dispatch_sync");
	}

	dispatch_lane_t dl = upcast(dq)._dl;
	// Global concurrent queues and queues bound to non-dispatch threads
	// always fall into the slow case, see DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE
	if (unlikely(!_dispatch_queue_try_reserve_sync_width(dl))) {
		return _dispatch_sync_f_slow(dl, ctxt, func, 0, dl, dc_flags);
	}

	if (unlikely(dq->do_targetq->do_targetq)) {
		return _dispatch_sync_recurse(dl, ctxt, func, dc_flags);
	}
	_dispatch_introspection_sync_begin(dl);
	_dispatch_sync_invoke_and_complete(dl, ctxt, func DISPATCH_TRACE_ARG(
			_dispatch_trace_item_sync_push_pop(dq, ctxt, func, dc_flags)));
}

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    注意这里，调用了_dispatch_barrier_sync_f这个从名字看，最终调用了栅栏函数。但是为什么要调用栅栏函数呢？我们先继续往里分析：

    static void
_dispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func, uintptr_t dc_flags)
{
	_dispatch_barrier_sync_f_inline(dq, ctxt, func, dc_flags);
}

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    来到 _dispatch_barrier_sync_f_inline，这里的参数func就是我们外面的block任务：

    static inline void
_dispatch_barrier_sync_f_inline(dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func, uintptr_t dc_flags)
{
	// 获取线程ID
	dispatch_tid tid = _dispatch_tid_self();

	if (unlikely(dx_metatype(dq) != _DISPATCH_LANE_TYPE)) {
		DISPATCH_CLIENT_CRASH(0, "Queue type doesn't support dispatch_sync");
	}

	dispatch_lane_t dl = upcast(dq)._dl;
	// 死锁
	if (unlikely(!_dispatch_queue_try_acquire_barrier_sync(dl, tid))) {
		return _dispatch_sync_f_slow(dl, ctxt, func, DC_FLAG_BARRIER, dl,
				DC_FLAG_BARRIER | dc_flags);
	}

	if (unlikely(dl->do_targetq->do_targetq)) {
		return _dispatch_sync_recurse(dl, ctxt, func,
				DC_FLAG_BARRIER | dc_flags);
	}
	_dispatch_introspection_sync_begin(dl);
	_dispatch_lane_barrier_sync_invoke_and_complete(dl, ctxt, func
			DISPATCH_TRACE_ARG(_dispatch_trace_item_sync_push_pop(
					dq, ctxt, func, dc_flags | DC_FLAG_BARRIER)));
}

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    在这个函数里会先获取线程 id，因为队列需要绑定到线程然后依赖执行，而死锁的原因在于同步线程里的任务出现你等我，我等你的现象，所以只有 _dispatch_queue_try_acquire_barrier_sync 用到了线程 id：

    static inline bool
_dispatch_queue_try_acquire_barrier_sync(dispatch_queue_class_t dq, uint32_t tid)
{
	return _dispatch_queue_try_acquire_barrier_sync_and_suspend(dq._dl, tid, 0);
}

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    static inline bool
_dispatch_queue_try_acquire_barrier_sync_and_suspend(dispatch_lane_t dq,
		uint32_t tid, uint64_t suspend_count)
{
	uint64_t init  = DISPATCH_QUEUE_STATE_INIT_VALUE(dq->dq_width);
	uint64_t value = DISPATCH_QUEUE_WIDTH_FULL_BIT | DISPATCH_QUEUE_IN_BARRIER |
			_dispatch_lock_value_from_tid(tid) |
			DISPATCH_QUEUE_UNCONTENDED_SYNC |
			(suspend_count * DISPATCH_QUEUE_SUSPEND_INTERVAL);
	uint64_t old_state, new_state;
	// 从 os 底层获取信息，也就是通过线程和当前队列获取 new_state 返回出去
	return os_atomic_rmw_loop2o(dq, dq_state, old_state, new_state, acquire, {
		uint64_t role = old_state & DISPATCH_QUEUE_ROLE_MASK;
		if (old_state != (init | role)) {
			os_atomic_rmw_loop_give_up(break);
		}
		new_state = value | role;
	});
}

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    从 os 底层获取到了一个 new_state 之后，就会继续执行 _dispatch_sync_f_slow：

    static void
_dispatch_sync_f_slow(dispatch_queue_class_t top_dqu, void *ctxt,
		dispatch_function_t func, uintptr_t top_dc_flags,
		dispatch_queue_class_t dqu, uintptr_t dc_flags)
{
	dispatch_queue_t top_dq = top_dqu._dq;
	dispatch_queue_t dq = dqu._dq;
	if (unlikely(!dq->do_targetq)) {
		return _dispatch_sync_function_invoke(dq, ctxt, func);
	}

	pthread_priority_t pp = _dispatch_get_priority();
	// 初始化保存 block 以及其他信息的结构体
	struct dispatch_sync_context_s dsc = {
		.dc_flags    = DC_FLAG_SYNC_WAITER | dc_flags,
		.dc_func     = _dispatch_async_and_wait_invoke,
		.dc_ctxt     = &dsc,
		.dc_other    = top_dq,
		.dc_priority = pp | _PTHREAD_PRIORITY_ENFORCE_FLAG,
		.dc_voucher  = _voucher_get(),
		.dsc_func    = func,
		.dsc_ctxt    = ctxt,
		.dsc_waiter  = _dispatch_tid_self(),
	};
	// 将 block push 到 queue 里面去
	_dispatch_trace_item_push(top_dq, &dsc);
	// 死锁的最终函数
	__DISPATCH_WAIT_FOR_QUEUE__(&dsc, dq);

	if (dsc.dsc_func == NULL) {
		// dsc_func being cleared means that the block ran on another thread ie.
		// case (2) as listed in _dispatch_async_and_wait_f_slow.
		dispatch_queue_t stop_dq = dsc.dc_other;
		return _dispatch_sync_complete_recurse(top_dq, stop_dq, top_dc_flags);
	}

	_dispatch_introspection_sync_begin(top_dq);
	_dispatch_trace_item_pop(top_dq, &dsc);
	_dispatch_sync_invoke_and_complete_recurse(top_dq, ctxt, func,top_dc_flags
			DISPATCH_TRACE_ARG(&dsc));
}

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    通过 _dispatch_trace_item_push 函数可以发现队列其实就是一个用来提交 block 的对象，当 block push 到队列中后，将按照 先入先出(FIFO) 的顺序进行处理，系统在 GCD 的底层会维护一个线程池，用来执行这些 block。关于死锁，我们一会儿再具体分析。
    block的大致调用流程：

    dispatch_sync
    _dispatch_sync_f
    _dispatch_sync_invoke_and_complete
    _dispatch_sync_function_invoke_inline
    _dispatch_client_callout
    f(ctxt);

    _dispatch_barrier_sync_f_inline的func直接传入到_dispatch_lane_barrier_sync_invoke_and_complete方法内部，接下来看一下：

    static void
_dispatch_lane_barrier_sync_invoke_and_complete(dispatch_lane_t dq,
		void *ctxt, dispatch_function_t func DISPATCH_TRACE_ARG(void *dc))
{
	_dispatch_sync_function_invoke_inline(dq, ctxt, func);
	...
}

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    static inline void
_dispatch_sync_function_invoke_inline(dispatch_queue_class_t dq, void *ctxt,
		dispatch_function_t func)
{
	dispatch_thread_frame_s dtf;
	_dispatch_thread_frame_push(&dtf, dq);
	// ⚠️⚠️使用了func
	_dispatch_client_callout(ctxt, func);
	_dispatch_perfmon_workitem_inc();
	_dispatch_thread_frame_pop(&dtf);
}

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    void
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
	_dispatch_get_tsd_base();
	void *u = _dispatch_get_unwind_tsd();
	if (likely(!u)) return f(ctxt);
	_dispatch_set_unwind_tsd(NULL);
	// ⚠️⚠️
	f(ctxt);
	_dispatch_free_unwind_tsd();
	_dispatch_set_unwind_tsd(u);
}

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    终于来到了最我们block的执行的地方，这里可以看到block是直接执行了，所以遇到同步函数，我们可以粗暴的理解为，里面的任务马上就要执行。看完了block的执行步骤，我们接下来看一下死锁的具体函数。

    死锁

    static void
__DISPATCH_WAIT_FOR_QUEUE__(dispatch_sync_context_t dsc, dispatch_queue_t dq)
{
	uint64_t dq_state = _dispatch_wait_prepare(dq);
	if (unlikely(_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter))) {
		DISPATCH_CLIENT_CRASH((uintptr_t)dq_state,
				"dispatch_sync called on queue "
				"already owned by current thread");
	}

	// Blocks submitted to the main thread MUST run on the main thread, and
	// dispatch_async_and_wait also executes on the remote context rather than
	// the current thread.
	//
	// For both these cases we need to save the frame linkage for the sake of
	// _dispatch_async_and_wait_invoke
	
	// 提交到主线程的块必须在主线程上运行，并且
	// dispatch_async_and_wait也在远程上下文而不是当前线程上执行。
	//
	// 对于这两种情况，我们需要保存框架链接，以便_dispatch_async_and_wait_invoke
	_dispatch_thread_frame_save_state(&dsc->dsc_dtf);

	...
}

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    可以看到如果满足_dq_state_drain_locked_by的条件就会触发crash。再看条件中函数的实现：

    static inline bool
_dq_state_drain_locked_by(uint64_t dq_state, dispatch_tid tid)
{
	return _dispatch_lock_is_locked_by((dispatch_lock)dq_state, tid);
}

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    static inline bool
_dispatch_lock_is_locked_by(dispatch_lock lock_value, dispatch_tid tid)
{
	// equivalent to _dispatch_lock_owner(lock_value) == tid
	// lock_value 为队列状态，tid 为线程 id
	// ^ (异或运算法) 两个相同就会出现 0 否则为1
	return ((lock_value ^ tid) & DLOCK_OWNER_MASK) == 0;
}

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    就是相当于当dispatch_sync + 串行队列的时候，这个串行队列就会对应一个线程，如果添加任务的代码执行的线程，和串行队列所对应的线程是一个线程的时候，就会发生死锁，从而crash。

    栅栏函数

    现在解决前面的问题，为什么会调用barrier函数？
    如果是并发队列，岂不是会执行完之前的任务，才会执行当前任务么？
    请添加图片描述
    请添加图片描述
    发现并没有走barrier函数。再次回到barrier调用的源码：

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

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    可以看到barrier调用的条件是dq_width == 1，上文我们也了解到，只有串行队列的dq_width才为1，故如果是串行队列走上面的分支，如果是并发队列走下面的分支，最终调用：

    _dispatch_sync_invoke_and_complete(dl, ctxt, func DISPATCH_TRACE_ARG(
			_dispatch_trace_item_sync_push_pop(dq, ctxt, func, dc_flags)));

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    可以理解为在串行队列中，同步函数相同于栅栏函数，会等待队列中之前的任务完成之后再执行当前任务，就是只有一个队伍，不可以插队，先来后到。并行队列中，允许排几个队伍，大家各个队伍互不干扰，并不会等之前的任务执行完成，再执行这个同步任务，而是优先执行同步任务。

    dispatch_async 异步源码分析

    异步函数会需要开启线程去执行任务，所以这应该会是一个重点。先看实现：

    void
dispatch_async(dispatch_queue_t dq, dispatch_block_t work)
{
	dispatch_continuation_t dc = _dispatch_continuation_alloc();
	uintptr_t dc_flags = DC_FLAG_CONSUME;
	dispatch_qos_t qos;

	qos = _dispatch_continuation_init(dc, dq, work, 0, dc_flags);
	_dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}

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    异步函数会通过 _dispatch_continuation_init 先对 block 进行包装即函数式保存，看一下代码：

    static inline dispatch_qos_t
_dispatch_continuation_init(dispatch_continuation_t dc,
		dispatch_queue_class_t dqu, dispatch_block_t work,
		dispatch_block_flags_t flags, uintptr_t dc_flags)
{
	void *ctxt = _dispatch_Block_copy(work);

	dc_flags |= DC_FLAG_BLOCK | DC_FLAG_ALLOCATED;
	if (unlikely(_dispatch_block_has_private_data(work))) {
		dc->dc_flags = dc_flags;
		dc->dc_ctxt = ctxt;
		// will initialize all fields but requires dc_flags & dc_ctxt to be set
		return _dispatch_continuation_init_slow(dc, dqu, flags);
	}

	dispatch_function_t func = _dispatch_Block_invoke(work);
	if (dc_flags & DC_FLAG_CONSUME) {
		func = _dispatch_call_block_and_release;
	}
	return _dispatch_continuation_init_f(dc, dqu, ctxt, func, flags, dc_flags);
}

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    把block封装成了dispatch_function_t类型的func，之后又进一步进行了处理。接下来继续看有关work的处理。

    static inline void
_dispatch_continuation_async(dispatch_queue_class_t dqu,
		dispatch_continuation_t dc, dispatch_qos_t qos, uintptr_t dc_flags)
{
#if DISPATCH_INTROSPECTION
	if (!(dc_flags & DC_FLAG_NO_INTROSPECTION)) {
		_dispatch_trace_item_push(dqu, dc);
	}
#else
	(void)dc_flags;
#endif
	return dx_push(dqu._dq, dc, qos);
}

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    注意这里的dx_push，在宏定义中找到它的定义：

    #define dx_push(x, y, z) dx_vtable(x)->dq_push(x, y, z)

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    请添加图片描述
    这里是根据不同种类的队列，执行不同的函数。通过观察不难发现，dx_push(dqu._dq, dc, qos);里的dqu其实就是我们传进来的队列。
    我们通过打断点的方式查看block的调用流程：
    请添加图片描述
    调用了_dispatch_worker_thread2函数，该函数只在_dispatch_root_queues_init_once里面被调用过，而它也只在_dispatch_root_queues_init函数里面被调用。继续寻找，_dispatch_root_queues_init在_dispatch_root_queue_poke_slow函数里被调用。这个方法是一个重点。_dispatch_root_queue_poke_slow函数是_dispatch_root_queue_poke的返回值，而_dispatch_root_queue_poke在_dispatch_root_queue_push_inline里面有机会被调用，这个inline函数明显由_dispatch_root_queue_push调用。我也看了很多别人的博客，大概对这个过程有了一点理解，接下来使用并发队列举例。

    Block的调用流程

    并发队列_dispatch_lane_concurrent_push：

    void
_dispatch_lane_concurrent_push(dispatch_lane_t dq, dispatch_object_t dou,
		dispatch_qos_t qos)
{
	if (dq->dq_items_tail == NULL &&
			!_dispatch_object_is_waiter(dou) &&
			!_dispatch_object_is_barrier(dou) &&
			_dispatch_queue_try_acquire_async(dq)) {
		// 非栅栏情况走_redirect_push
		return _dispatch_continuation_redirect_push(dq, dou, qos);
	}
	// 其他情况走
	_dispatch_lane_push(dq, dou, qos);
}

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    非栅栏情况，接着走_dispatch_continuation_redirect_push：

    static void
_dispatch_continuation_redirect_push(dispatch_lane_t dl,
		dispatch_object_t dou, dispatch_qos_t qos)
{
	if (likely(!_dispatch_object_is_redirection(dou))) {
		dou._dc = _dispatch_async_redirect_wrap(dl, dou);
	} else if (!dou._dc->dc_ctxt) {
		// find first queue in descending target queue order that has
		// an autorelease frequency set, and use that as the frequency for
		// this continuation.
		dou._dc->dc_ctxt = (void *)
		(uintptr_t)_dispatch_queue_autorelease_frequency(dl);
	}
	dispatch_queue_t dq = dl->do_targetq;
	if (!qos) qos = _dispatch_priority_qos(dq->dq_priority);
	dx_push(dq, dou, qos);// 又做了一遍dx_push,此时的入参dq === do_do_targetq
	// 原因在于GCD也是对象，也存在继承封装的问题，类似于 类 父类 根类的关系。
}

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    这里会发现又走到了dx_push，即递归了，综合前面队列创建时可知，队列也是一个对象，有父类、根类，所以会递归执行到根类的方法。
    那do_targetq是什么呢？得回到队列的创建dispatch_queue_create去查看：
    请添加图片描述
    请添加图片描述
    tq是root_queue。
    请添加图片描述
    将tq赋值给do_targetq，所以do_targetq就是root_queue。
    请添加图片描述
    再看root_queue的类型是dispatch_queue_global_t，所以说_dispatch_continuation_redirect_push里的dx_push时的队列是dispatch_queue_global_t，我们需要去找对应的dq_push 的方法：

    void
_dispatch_root_queue_push(dispatch_queue_global_t rq, dispatch_object_t dou,
		dispatch_qos_t qos)
{
#if DISPATCH_USE_KEVENT_WORKQUEUE
	dispatch_deferred_items_t ddi = _dispatch_deferred_items_get();
	if (unlikely(ddi && ddi->ddi_can_stash)) {
		dispatch_object_t old_dou = ddi->ddi_stashed_dou;
		dispatch_priority_t rq_overcommit;
		rq_overcommit = rq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;

		if (likely(!old_dou._do || rq_overcommit)) {
			dispatch_queue_global_t old_rq = ddi->ddi_stashed_rq;
			dispatch_qos_t old_qos = ddi->ddi_stashed_qos;
			ddi->ddi_stashed_rq = rq;
			ddi->ddi_stashed_dou = dou;
			ddi->ddi_stashed_qos = qos;
			_dispatch_debug("deferring item %p, rq %p, qos %d",
					dou._do, rq, qos);
			if (rq_overcommit) {
				ddi->ddi_can_stash = false;
			}
			if (likely(!old_dou._do)) {
				return;
			}
			// push the previously stashed item
			qos = old_qos;
			rq = old_rq;
			dou = old_dou;
		}
	}
#endif
#if HAVE_PTHREAD_WORKQUEUE_QOS
	if (_dispatch_root_queue_push_needs_override(rq, qos)) {
		return _dispatch_root_queue_push_override(rq, dou, qos);
	}
#else
	(void)qos;
#endif
	_dispatch_root_queue_push_inline(rq, dou, dou, 1);
}

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    进去_dispatch_root_queue_push_inline：

    static inline void
_dispatch_root_queue_push_inline(dispatch_queue_global_t dq,
		dispatch_object_t _head, dispatch_object_t _tail, int n)
{
	struct dispatch_object_s *hd = _head._do, *tl = _tail._do;
	if (unlikely(os_mpsc_push_list(os_mpsc(dq, dq_items), hd, tl, do_next))) {
		return _dispatch_root_queue_poke(dq, n, 0);
	}
}

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    进到_dispatch_root_queue_poke：

    void
_dispatch_root_queue_poke(dispatch_queue_global_t dq, int n, int floor)
{
	if (!_dispatch_queue_class_probe(dq)) {
		return;
	}
#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
	if (likely(dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE))
#endif
	{
		if (unlikely(!os_atomic_cmpxchg2o(dq, dgq_pending, 0, n, relaxed))) {
			_dispatch_root_queue_debug("worker thread request still pending "
					"for global queue: %p", dq);
			return;
		}
	}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
	return _dispatch_root_queue_poke_slow(dq, n, floor);
}

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    接下来就到了我们前边提到过的_dispatch_root_queue_poke_slow函数。

    _dispatch_root_queue_poke_slow

    static void
_dispatch_root_queue_poke_slow(dispatch_queue_global_t dq, int n, int floor)
{
	int remaining = n;
#if !defined(_WIN32)
	int r = ENOSYS;
#endif

	_dispatch_root_queues_init();
	_dispatch_debug_root_queue(dq, __func__);
	_dispatch_trace_runtime_event(worker_request, dq, (uint64_t)n);

#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_ROOT_QUEUES
	if (dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE)
	// 如果是全局队列，那么就会创建线程去处理。
#endif
	{
		_dispatch_root_queue_debug("requesting new worker thread for global "
				"queue: %p", dq);
		r = _pthread_workqueue_addthreads(remaining,
				_dispatch_priority_to_pp_prefer_fallback(dq->dq_priority));
		(void)dispatch_assume_zero(r);
		return;
	}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
	dispatch_pthread_root_queue_context_t pqc = dq->do_ctxt;
	if (likely(pqc->dpq_thread_mediator.do_vtable)) {
		while (dispatch_semaphore_signal(&pqc->dpq_thread_mediator)) {
			_dispatch_root_queue_debug("signaled sleeping worker for "
					"global queue: %p", dq);
			if (!--remaining) {
				return;
			}
		}
	}

	bool overcommit = dq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
	if (overcommit) {
		// 串行队列
		os_atomic_add2o(dq, dgq_pending, remaining, relaxed);
	} else {
		if (!os_atomic_cmpxchg2o(dq, dgq_pending, 0, remaining, relaxed)) {
			_dispatch_root_queue_debug("worker thread request still pending for "
					"global queue: %p", dq);
			return;
		}
	}
	// floor 为 0，remaining 是根据队列任务的情况处理的
	int can_request, t_count;
	// seq_cst with atomic store to tail 
	// 获取线程池的大小
	t_count = os_atomic_load2o(dq, dgq_thread_pool_size, ordered);
	// 如果是普通的自己创建的线程，就会进行dowhile循环。
	// 这里dgq_thread_pool_size会暂时标记为1，这是因为正常的并行队列是0的，而全局队列为1的是因为他的线程量比正常的并行队列多一个。
	/*
	static const struct dispatch_queue_global_s _dispatch_custom_workloop_root_queue = {
		DISPATCH_GLOBAL_OBJECT_HEADER(queue_global),
		.dq_state = DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE,
		.do_ctxt = NULL,
		.dq_label = "com.apple.root.workloop-custom",
		.dq_atomic_flags = DQF_WIDTH(DISPATCH_QUEUE_WIDTH_POOL),
		.dq_priority = _dispatch_priority_make_fallback(DISPATCH_QOS_DEFAULT) |
			DISPATCH_PRIORITY_SATURATED_OVERRIDE,
		.dq_serialnum = DISPATCH_QUEUE_SERIAL_NUMBER_WLF,
		.dgq_thread_pool_size = 1,
	};
	*/
	do {
		// 计算可以请求的数量
		// t_count是通过os_atomic_load2o得来的
		// floor是之前传过来的参数
		can_request = t_count < floor ? 0 : t_count - floor;
		if (remaining > can_request) {
			_dispatch_root_queue_debug("pthread pool reducing request from %d to %d",
					remaining, can_request);
			os_atomic_sub2o(dq, dgq_pending, remaining - can_request, relaxed);
			remaining = can_request;
		}
		// remaining 一般不会大于 can_request，否则就会报异常。
		// remaining是需要的线程数，而can_request是可以请求的线程数。这里如果大于，就会进行--的操作，如果remaining为0，那么就代表着线程池已经满了，那么就会直接return
		if (remaining == 0) {
			// 线程池满了，就会报出异常的情况
			_dispatch_root_queue_debug("pthread pool is full for root queue: "
					"%p", dq);
			return;
		}
	} while (!os_atomic_cmpxchgv2o(dq, dgq_thread_pool_size, t_count,
			t_count - remaining, &t_count, acquire));

#if !defined(_WIN32)
	pthread_attr_t *attr = &pqc->dpq_thread_attr;
	pthread_t tid, *pthr = &tid;
#if DISPATCH_USE_MGR_THREAD && DISPATCH_USE_PTHREAD_ROOT_QUEUES
	if (unlikely(dq == &_dispatch_mgr_root_queue)) {
		pthr = _dispatch_mgr_root_queue_init();
	}
#endif
	do {
		_dispatch_retain(dq); // released in _dispatch_worker_thread
		// 开辟线程
		while ((r = pthread_create(pthr, attr, _dispatch_worker_thread, dq))) {
			if (r != EAGAIN) {
				(void)dispatch_assume_zero(r);
			}
			_dispatch_temporary_resource_shortage();
		}
	} while (--remaining);
#else // defined(_WIN32)
#if DISPATCH_USE_MGR_THREAD && DISPATCH_USE_PTHREAD_ROOT_QUEUES
	if (unlikely(dq == &_dispatch_mgr_root_queue)) {
		_dispatch_mgr_root_queue_init();
	}
#endif
	do {
		_dispatch_retain(dq); // released in _dispatch_worker_thread
#if DISPATCH_DEBUG
		unsigned dwStackSize = 0;
#else
		unsigned dwStackSize = 64 * 1024;
#endif
		uintptr_t hThread = 0;
		while (!(hThread = _beginthreadex(NULL, dwStackSize, _dispatch_worker_thread_thunk, dq, STACK_SIZE_PARAM_IS_A_RESERVATION, NULL))) {
			if (errno != EAGAIN) {
				(void)dispatch_assume(hThread);
			}
			_dispatch_temporary_resource_shortage();
		}
#if DISPATCH_USE_PTHREAD_ROOT_QUEUES
		if (_dispatch_mgr_sched.prio > _dispatch_mgr_sched.default_prio) {
			(void)dispatch_assume_zero(SetThreadPriority((HANDLE)hThread, _dispatch_mgr_sched.prio) == TRUE);
		}
#endif
		CloseHandle((HANDLE)hThread);
	} while (--remaining);
#endif // defined(_WIN32)
#else
	(void)floor;
#endif // DISPATCH_USE_PTHREAD_POOL
}

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    里面先调用了_dispatch_root_queues_init()，看一下：

    static inline void
_dispatch_root_queues_init(void)
{
	dispatch_once_f(&_dispatch_root_queues_pred, NULL,
			_dispatch_root_queues_init_once);
}

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    这里进行了单例的调用，调用了_dispatch_root_queues_init_once，看一下，里面可以看到这：
    请添加图片描述
    上文提到过的_dispatch_worker_thread2在这里封装给了pthread的api。GCD也是对pthread的封装。这里的调用执行，是通过workloop调用的，而不是立即执行，是受cpu进行调控处理的。

    semaphore 信号量源码分析

    信号量的API不多，dispatch_semaphore_t的常用方法有三个：

    • dispatch_semaphore_create 创建信号量
    • dispatch_semaphore_wait 等待信号量
    • dispatch_semaphore_signal 释放信号量

    dispatch_semaphore_create

    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.
	// 初始值必须大于等于0
	if (value < 0) {
		return DISPATCH_BAD_INPUT;
	}
	//开辟内存
	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);
	//保存初始值
	dsema->dsema_value = value;
	//初始化方法
	_dispatch_sema4_init(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
	dsema->dsema_orig = value;
	return dsema;
}

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    如果信号为小于0，则返回一个DISPATCH_BAD_INPUT类型对象，也就是返回个_Nonnull（#define DISPATCH_BAD_INPUT ((void *_Nonnull)0)）
    如果信号大于等于0，就会dispatch_semaphore_t对象dsema进行初始化，并返回dsema对象。

    dispatch_semaphore_wait

    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);
}

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    dispatch_atomic_dec2o 是一个宏，会调用 GCC 内置的函数 __sync_sub_and_fetch，实现减法的原子性操作。因此意思是将 dsema 的值减一，并把新的值赋给 value。如果减一后的 value 大于等于 0 就立刻返回，没有任何操作，否则调用_dispatch_semaphore_wait_slow，_dispatch_semaphore_wait_slow 函数针对不同的 timeout 参数，分了三种情况考虑。

    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;
		}
		// Fall through and try to undo what the fast path did to
		// dsema->dsema_value
	case DISPATCH_TIME_NOW:
		orig = dsema->dsema_value;
		while (orig < 0) {
			if (os_atomic_cmpxchgv2o(dsema, dsema_value, orig, orig + 1,
					&orig, relaxed)) {
				return _DSEMA4_TIMEOUT();
			}
		}
		// Another thread called semaphore_signal().
		// Fall through and drain the wakeup.
	case DISPATCH_TIME_FOREVER:
		_dispatch_sema4_wait(&dsema->dsema_sema);
		break;
	}
	return 0;
}

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    第一种情况 while 判断一定会成立，因为如果 value 大于等于 0，在上一个函数 dispatch_semaphore_wait 中就已经返回了，判断成立，内部的 if 判断一定也成立，此时会将 value 加一（也就是变为 0）并返回。加一的原因是为了抵消 wait 函数一开始的减一操作。此时函数调用方会得到返回值 KERN_OPERATION_TIMED_OUT，表示由于等待时间超时而返回。
    第二种情况是 DISPATCH_TIME_FOREVER，会调用系统的 semaphore_wait 方法继续等待，直到收到 signal 调用。
    第三种就是一个默认的情况，在default 分支下，我们指定一个超时时间，在 _dispatch_sema4_timedwait 里面会去判断当前操作是否超时，这和 DISPATCH_TIME_FOREVER 的处理比较类似，不同的是我们调用了内核提供的 semaphore_timedwait 方法可以指定超时时间。
    接下来看一下常用的 DISPATCH_TIME_FOREVER情况下调用的_dispatch_sema4_wait方法：

    void
_dispatch_sema4_wait(_dispatch_sema4_t *sema)
{
	kern_return_t kr;
	do {
		kr = semaphore_wait(*sema);
	} while (kr == KERN_ABORTED);
	DISPATCH_SEMAPHORE_VERIFY_KR(kr);
}

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    这里进行了do-while循环，这个循环就相当于忙等。

    dispatch_semaphore_signal

    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
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;
}

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    • 首先信号量的值+1
    • 如果加完，值大于0，直接返回。
    • 否则执行 _dispatch_semaphore_signal_slow

    dispatch_once一次性代码

    其实，这个一次性代码就是我们经常使用的dispatch_once方法。我们经常在只需要执行一次的代码上使用它（比如说单例）。

    void
dispatch_once(dispatch_once_t *val, dispatch_block_t block)
{
	dispatch_once_f(val, block, _dispatch_Block_invoke(block));
}
void
dispatch_once_f(dispatch_once_t *val, void *ctxt, dispatch_function_t func)
{
	dispatch_once_gate_t l = (dispatch_once_gate_t)val;

#if !DISPATCH_ONCE_INLINE_FASTPATH || DISPATCH_ONCE_USE_QUIESCENT_COUNTER
	uintptr_t v = os_atomic_load(&l->dgo_once, acquire);
	// 如果标识符是DLOCK_ONCE_DONE，代表已经执行过，那么就直接return
	if (likely(v == DLOCK_ONCE_DONE)) {
		return;
	}
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
	//已经完成分支
	if (likely(DISPATCH_ONCE_IS_GEN(v))) {
		return _dispatch_once_mark_done_if_quiesced(l, v);
	}
#endif
#endif
	//未执行过分支
	if (_dispatch_once_gate_tryenter(l)) {
		return _dispatch_once_callout(l, ctxt, func);
	}
	//正在别的线程执行分支
	return _dispatch_once_wait(l);
}

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    先看一看宏定义：

    #if defined(__x86_64__) || defined(__i386__) || defined(__s390x__)
#define DISPATCH_ONCE_USE_QUIESCENT_COUNTER 0
#elif __APPLE__
#define DISPATCH_ONCE_USE_QUIESCENT_COUNTER 1
#else
#define DISPATCH_ONCE_USE_QUIESCENT_COUNTER 0
#endif

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    #if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
#define DISPATCH_ONCE_MAKE_GEN(gen)  (((gen) << 2) + DLOCK_FAILED_TRYLOCK_BIT)
#define DISPATCH_ONCE_IS_GEN(gen)    (((gen) & 3) == DLOCK_FAILED_TRYLOCK_BIT)

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    看设置已经完成的宏，是在callout流程里面调用的左移2位，然后 + 0x10(DLOCK_FAILED_TRYLOCK_BIT)，如果执行过次操作，下面的判断一定是YES，下面的判断是与0x11，即获取低2位值，看是否等于0x10。
    第一次进来的话，会走到_dispatch_once_gate_tryenter这里，而里面做了解锁的操作，是对多线程的封装处理，所以是线程安全的。最后，就调用了_dispatch_once_callout。如果加锁了，那么就会调用_dispatch_once_wait进行无限制等待开锁的状态。

    static inline bool
_dispatch_once_gate_tryenter(dispatch_once_gate_t l)
{
	return os_atomic_cmpxchg(&l->dgo_once, DLOCK_ONCE_UNLOCKED,
			(uintptr_t)_dispatch_lock_value_for_self(), relaxed);
}

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    这里主要是通过底层os_atomic_cmpxchg方法进行对比，如果比较没有问题，则进行加锁，即任务的标识符置为DLOCK_ONCE_UNLOCKED。接下来看一下_dispatch_once_callout：

    static void
_dispatch_once_callout(dispatch_once_gate_t l, void *ctxt,
		dispatch_function_t func)
{
	//执行block
	_dispatch_client_callout(ctxt, func);
	//设置执行状态
	_dispatch_once_gate_broadcast(l);
}

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    _dispatch_once_callout调用_dispatch_client_callout执行了block，调用_dispatch_once_gate_broadcast进行标记符的处理。

    #undef _dispatch_client_callout
DISPATCH_NOINLINE
void
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
	_dispatch_get_tsd_base();
	void *u = _dispatch_get_unwind_tsd();
	if (likely(!u)) return f(ctxt);
	_dispatch_set_unwind_tsd(NULL);
	f(ctxt);
	_dispatch_free_unwind_tsd();
	_dispatch_set_unwind_tsd(u);
}

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    接下来再看一下_dispatch_once_gate_broadcast的实现：

    static inline void
_dispatch_once_gate_broadcast(dispatch_once_gate_t l)
{
	dispatch_lock value_self = _dispatch_lock_value_for_self();
	uintptr_t v;
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
	//获取已经完成的值
	v = _dispatch_once_mark_quiescing(l);
#else
	v = _dispatch_once_mark_done(l);
#endif
	if (likely((dispatch_lock)v == value_self)) return;
	_dispatch_gate_broadcast_slow(&l->dgo_gate, (dispatch_lock)v);
}

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    static inline uintptr_t
_dispatch_once_mark_quiescing(dispatch_once_gate_t dgo)
{
	//线程安全
	return os_atomic_xchg(&dgo->dgo_once, _dispatch_once_generation(), release);
}

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    DISPATCH_ONCE_MAKE_GEN便是设置已经完成的值。

    group 源码分析

    接下来浅浅了解下与group相关的源码。

    dispatch_group_create

    先看看如何创建 group：

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

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    这个方法就是开辟了内存空间，但是从 os_atomic_store2o 可以看出 group 底层也维护了一个 value 值。

    dispatch_group_enter

    从 enter 可以看出，当进组的时候会通过 os_atomic_sub_orig2o 对 value 减 4。

    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); // 
	}
	if (unlikely(old_value == DISPATCH_GROUP_VALUE_MAX)) {
		DISPATCH_CLIENT_CRASH(old_bits,
				"Too many nested calls to dispatch_group_enter()");
	}
}

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    dispatch_group_leave

    出组的时候会对 value 进行加值，如果 new_state 和 old_state 相等，就会调用 _dispatch_group_wake 继续后面代码的执行

    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()");
	}
}

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    dispatch_group_async

    dispatch_group_async 就是对 enter 和 leave 的封装，当 block 调用完成之后进行 callout 之后就出组了。

    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;
	// 保存任务 
	qos = _dispatch_continuation_init(dc, dq, db, 0, dc_flags);
	_dispatch_continuation_group_async(dg, dq, dc, qos);
}

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

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    static inline void
_dispatch_continuation_with_group_invoke(dispatch_continuation_t dc)
{
    struct dispatch_object_s *dou = dc->dc_data;
    unsigned long type = dx_type(dou);
    if (type == DISPATCH_GROUP_TYPE) {
    	_dispatch_client_callout(dc->dc_ctxt, dc->dc_func);
    	_dispatch_trace_item_complete(dc);
    	// 出组
    	dispatch_group_leave((dispatch_group_t)dou);
    } else {
    	DISPATCH_INTERNAL_CRASH(dx_type(dou), "Unexpected object type");
    }
}

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    dispatch_group_wait

    这个方法用于等待 group 中所有任务执行完成，可以理解为信号量 wait 的封装

    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);
}

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    如果当前 value 和原始 value 相同，表明任务已经全部完成，直接返回 0，如果 timeout 为 0 也会立刻返回，否则调用 _dispatch_group_wait_slow。
    在 _dispatch_group_wait_slow 会一直等到任务完成返回 0 ，当然如果一直没有完成就会返回 timeout。

    _dispatch_group_wake

    这个函数主要做的就是循环调用 dispatch_async_f 异步执行在 notify 函数中注册的回调。

    static void
_dispatch_group_wake(dispatch_group_t dg, uint64_t dg_state, bool needs_release)
{
	uint16_t refs = needs_release ? 1 : 0; // 
	if (dg_state & DISPATCH_GROUP_HAS_NOTIFS) {
		dispatch_continuation_t dc, next_dc, tail;
		// Snapshot before anything is notified/woken 
		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);
}

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  • 原文地址：https://blog.csdn.net/weixin_52192405/article/details/125471159