同步函数死锁

死锁现象

• 主线程因为同步函数的原因等着先执⾏任务
• 主队列等着主线程的任务执⾏完毕再执⾏⾃⼰的任务
• 主队列和主线程相互等待会造成死锁

同步函数和异步函数的区别

• 能否开辟线程
• 任务的回调是否具备异步性或同步性

同步串行死锁底层源码分析

• 全局搜索dispatch_sync(dis

DISPATCH_NOINLINE
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);
}
#endif // __BLOCKS__

• 查看_dispatch_sync_f方法
• 查看_dispatch_sync_f_inline方法

DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_sync_f_inline(dispatch_queue_t dq, void *ctxt,
        dispatch_function_t func, uintptr_t dc_flags)
{
    if (likely(dq->dq_width == 1)) {
        // 查看死锁方法
        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)));
}

• 查看_dispatch_barrier_sync_f方法
• 查看_dispatch_barrier_sync_f_inline方法

DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_barrier_sync_f_inline(dispatch_queue_t dq, void *ctxt,
        dispatch_function_t func, uintptr_t dc_flags)
{
    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;
    
    if (unlikely(!_dispatch_queue_try_acquire_barrier_sync(dl, tid))) {
        //查看_dispatch_sync_f_slow方法，死锁的时候会报这个异常
        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)));
}

死锁时报错

死锁真正报错的方法是__DISPATCH_WAIT_FOR_QUEUE__

image.png

• 查看_dispatch_sync_f_slow方法
• 查看__DISPATCH_WAIT_FOR_QUEUE__

DISPATCH_NOINLINE
static void
__DISPATCH_WAIT_FOR_QUEUE__(dispatch_sync_context_t dsc, dispatch_queue_t dq)
{
    uint64_t dq_state = _dispatch_wait_prepare(dq);
    // 查看死锁条件
    if (unlikely(_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter))) {
        // 死锁产生在这里
        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
    _dispatch_thread_frame_save_state(&dsc->dsc_dtf);

    if (_dq_state_is_suspended(dq_state) ||
            _dq_state_is_base_anon(dq_state)) {
        dsc->dc_data = DISPATCH_WLH_ANON;
    } else if (_dq_state_is_base_wlh(dq_state)) {
        dsc->dc_data = (dispatch_wlh_t)dq;
    } else {
        _dispatch_wait_compute_wlh(upcast(dq)._dl, dsc);
    }
...... //省略代码

里面的日志和我们崩溃的死锁最后执行__DISPATCH_WAIT_FOR_QUEUE__的截图日志是一样的，意味着死锁就在这里，你调用的队列被当前线程持有

• 产生死锁的dsc_waiter是从外面传进来的，返回去查看
  

  image.png

• 查看_dispatch_tid_self()是tid

#define _dispatch_tid_self()        ((dispatch_tid)_dispatch_thread_port())

• 由死锁判断条件if (unlikely(_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter)))，去查看dq_state状态，调用的是_dq_state_drain_locked_by方法

DISPATCH_ALWAYS_INLINE
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);
}

• 查看_dispatch_lock_is_locked_by方法

DISPATCH_ALWAYS_INLINE
static inline bool
_dispatch_lock_is_locked_by(dispatch_lock lock_value, dispatch_tid tid)
{
    // equivalent to _dispatch_lock_owner(lock_value) == tid
    return ((lock_value ^ tid) & DLOCK_OWNER_MASK) == 0;
}

lock_value = tid等于0，lock_value ^ tid = 0 & DLOCK_OWNER_MASK才等于0
dq_state和dsc->dsc_waiter代表这两个值相同才发生死锁

同步函数任务同步

同步函数全局并发队列底层源码分析

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

• _dispatch_sync_invoke_and_complete方法传参以func开头，为什么这么写？我们进去探索

DISPATCH_NOINLINE
static void
_dispatch_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);
    _dispatch_trace_item_complete(dc);
    _dispatch_lane_non_barrier_complete(dq, 0);
}

• 查看DISPATCH_TRACE_ARG方法

// 把逗号封装到这里，意味着arg是可选参数，根据条件来控制
#define DISPATCH_TRACE_ARG(arg) , arg

• 同步函数全局并发队列会执行到_dispatch_sync_f_inline方法中的哪个方法？我们不得而知，这里就添加符号断点来探索
  image.png

image.png

• 下面探索执行到_dispatch_sync_f_slow方法中的哪个方法？依然是添加符号断点来探索
  

  image.png

• 跳过断点继续执行，发现刚才添加的两个符号断点并没有执行到

image.png

• 那么应该就是执行到_dispatch_sync_f_slow方法中的_dispatch_sync_function_invoke，我们重新添加符号断点发现确实执行到这里。

DISPATCH_NOINLINE
static void
_dispatch_sync_function_invoke(dispatch_queue_class_t dq, void *ctxt,
        dispatch_function_t func)
{
    _dispatch_sync_function_invoke_inline(dq, ctxt, func);
}

• 查看_dispatch_sync_function_invoke_inline方法，执行的是_dispatch_client_callout

DISPATCH_ALWAYS_INLINE
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);
    _dispatch_client_callout(ctxt, func);
    _dispatch_perfmon_workitem_inc();
    _dispatch_thread_frame_pop(&dtf);
}

• bt查看顿栈信息

image.png

异步函数分析上

异步函数并发队列底层源码分析

• 查看dispatch_async方法

#ifdef __BLOCKS__
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);
}
#endif

• 查看_dispatch_continuation_async方法
• 查看dx_push方法

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

• 查看dq_push方法
  

  image.png

• 查看_dispatch_lane_concurrent_push方法

DISPATCH_NOINLINE
void
_dispatch_lane_concurrent_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
    // <rdar://problem/24738102&24743140> reserving non barrier width
    // doesn't fail if only the ENQUEUED bit is set (unlike its barrier
    // width equivalent), so we have to check that this thread hasn't
    // enqueued anything ahead of this call or we can break ordering
    if (dq->dq_items_tail == NULL &&
            !_dispatch_object_is_waiter(dou) &&
            // barrier表示栅栏，控制流程
            !_dispatch_object_is_barrier(dou) &&
            _dispatch_queue_try_acquire_async(dq)) {
        return _dispatch_continuation_redirect_push(dq, dou, qos);
    }

    _dispatch_lane_push(dq, dou, qos);
}

• 查看_dispatch_lane_push方法，可以添加符号断点查看执行流程

DISPATCH_NOINLINE
void
_dispatch_lane_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
    dispatch_wakeup_flags_t flags = 0;
    struct dispatch_object_s *prev;

    if (unlikely(_dispatch_object_is_waiter(dou))) {
        return _dispatch_lane_push_waiter(dq, dou._dsc, qos);
    }

    dispatch_assert(!_dispatch_object_is_global(dq));
    qos = _dispatch_queue_push_qos(dq, qos);

    prev = os_mpsc_push_update_tail(os_mpsc(dq, dq_items), dou._do, do_next);
    if (unlikely(os_mpsc_push_was_empty(prev))) {
        _dispatch_retain_2_unsafe(dq);
        flags = DISPATCH_WAKEUP_CONSUME_2 | DISPATCH_WAKEUP_MAKE_DIRTY;
    } else if (unlikely(_dispatch_queue_need_override(dq, qos))) {
        _dispatch_retain_2_unsafe(dq);
        flags = DISPATCH_WAKEUP_CONSUME_2;
    }
    os_mpsc_push_update_prev(os_mpsc(dq, dq_items), prev, dou._do, do_next);
    if (flags) {
        return dx_wakeup(dq, qos, flags);
    }
}

• 查看dx_wakeup方法

#define dx_wakeup(x, y, z) dx_vtable(x)->dq_wakeup(x, y, z)

• 查看dq_wakeup方法，执行的是_dispatch_lane_wakeup方法

DISPATCH_VTABLE_SUBCLASS_INSTANCE(queue_concurrent, lane,
    .do_type        = DISPATCH_QUEUE_CONCURRENT_TYPE,
    .do_dispose     = _dispatch_lane_dispose,
    .do_debug       = _dispatch_queue_debug,
    .do_invoke      = _dispatch_lane_invoke,

    .dq_activate    = _dispatch_lane_activate,
    .dq_wakeup      = _dispatch_lane_wakeup,
    .dq_push        = _dispatch_lane_concurrent_push,
);

• 查看_dispatch_lane_wakeup方法
• 查看_dispatch_queue_wakeup方法

DISPATCH_NOINLINE
void
_dispatch_queue_wakeup(dispatch_queue_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags, dispatch_queue_wakeup_target_t target)
{
    dispatch_queue_t dq = dqu._dq;
    uint64_t old_state, new_state, enqueue = DISPATCH_QUEUE_ENQUEUED;
    dispatch_assert(target != DISPATCH_QUEUE_WAKEUP_WAIT_FOR_EVENT);

    if (target && !(flags & DISPATCH_WAKEUP_CONSUME_2)) {
        _dispatch_retain_2(dq);
        flags |= DISPATCH_WAKEUP_CONSUME_2;
    }

    if (unlikely(flags & DISPATCH_WAKEUP_BARRIER_COMPLETE)) {
        dispatch_assert(dx_metatype(dq) == _DISPATCH_SOURCE_TYPE);
        qos = _dispatch_queue_wakeup_qos(dq, qos);
        // 执行到_dispatch_lane_class_barrier_complete方法
        return _dispatch_lane_class_barrier_complete(upcast(dq)._dl, qos,
                flags, target, DISPATCH_QUEUE_SERIAL_DRAIN_OWNED);
    }
...... //省略代码

• 查看_dispatch_lane_class_barrier_complete方法

DISPATCH_NOINLINE
static void
_dispatch_lane_class_barrier_complete(dispatch_lane_t dq, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags, dispatch_queue_wakeup_target_t target,
        uint64_t owned)
{
    uint64_t old_state, new_state, enqueue;
    dispatch_queue_t tq;

    if (target == DISPATCH_QUEUE_WAKEUP_MGR) {
        tq = _dispatch_mgr_q._as_dq;
        enqueue = DISPATCH_QUEUE_ENQUEUED_ON_MGR;
    } else if (target) {
        tq = (target == DISPATCH_QUEUE_WAKEUP_TARGET) ? dq->do_targetq : target;
        enqueue = DISPATCH_QUEUE_ENQUEUED;
    } else {
        tq = NULL;
        enqueue = 0;
    }

again:
    // os_atomic_rmw_loop2o产生递归调用
    os_atomic_rmw_loop2o(dq, dq_state, old_state, new_state, release, {
        if (unlikely(_dq_state_needs_ensure_ownership(old_state))) {
            _dispatch_event_loop_ensure_ownership((dispatch_wlh_t)dq);
            _dispatch_queue_move_to_contended_sync(dq->_as_dq);
            os_atomic_rmw_loop_give_up(goto again);
        }
...... // 省略代码

if (_dq_state_received_override(old_state)) {
        // Ensure that the root queue sees that this thread was overridden.
        _dispatch_set_basepri_override_qos(_dq_state_max_qos(old_state));
    }

    if (tq) {
        if (likely((old_state ^ new_state) & enqueue)) {
            dispatch_assert(_dq_state_is_enqueued(new_state));
            dispatch_assert(flags & DISPATCH_WAKEUP_CONSUME_2);
            // 这里最终会执行到_dispatch_root_queue_push
            return _dispatch_queue_push_queue(tq, dq, new_state);
        }
#if HAVE_PTHREAD_WORKQUEUE_QOS
        if (_dq_state_should_override(new_state)) {
            return _dispatch_queue_wakeup_with_override(dq, new_state, flags);
        }
#endif
...... // 省略代码

• 查看_dispatch_root_queue_push方法
• 查看_dispatch_root_queue_push_inline方法

DISPATCH_ALWAYS_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);
    }
}

• 查看_dispatch_root_queue_poke方法
• 查看_dispatch_root_queue_poke_slow方法

DISPATCH_NOINLINE
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
     //  单例底层方法
    _dispatch_root_queues_init();
    _dispatch_debug_root_queue(dq, __func__);
    _dispatch_trace_runtime_event(worker_request, dq, (uint64_t)n);
...... //省略代码

_dispatch_root_queue_poke_slow方法中调用了_dispatch_root_queues_init，异步函数分析先暂停到这里，下面先分析单例底层原理......

单例底层原理

• 查看_dispatch_root_queues_init方法

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

GCD单例写法

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
});

• 查看dispatch_once方法

#ifdef __BLOCKS__
void
dispatch_once(dispatch_once_t *val, dispatch_block_t block)
{  
    // 用val做一个判断，来创建单例
    dispatch_once_f(val, block, _dispatch_Block_invoke(block));
}
#endif

• 查看dispatch_once_f方法

DISPATCH_NOINLINE
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);
    if (likely(v == DLOCK_ONCE_DONE)) {
        return;
    }
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
    if (likely(DISPATCH_ONCE_IS_GEN(v))) {
        return _dispatch_once_mark_done_if_quiesced(l, v);
    }
#endif
#endif
    // 添加锁
    if (_dispatch_once_gate_tryenter(l)) {
        return _dispatch_once_callout(l, ctxt, func);
    }
    // _dispatch_once_wait 等待检测 DLOCK_ONCE_DONE
    return _dispatch_once_wait(l);
}

• _dispatch_once_gate_tryenter上锁

DISPATCH_ALWAYS_INLINE
static inline bool
_dispatch_once_gate_tryenter(dispatch_once_gate_t l)
{
    // 原子操作，线程锁
    return os_atomic_cmpxchg(&l->dgo_once, DLOCK_ONCE_UNLOCKED,
            (uintptr_t)_dispatch_lock_value_for_self(), relaxed);
}

• 查看_dispatch_once_callout方法，回调block

DISPATCH_NOINLINE
static void
_dispatch_once_callout(dispatch_once_gate_t l, void *ctxt,
        dispatch_function_t func)
{
    _dispatch_client_callout(ctxt, func);
    _dispatch_once_gate_broadcast(l);
}

• _dispatch_once_gate_broadcast方法解锁

DISPATCH_ALWAYS_INLINE
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
    v = _dispatch_once_mark_quiescing(l);
#else
    // 设值done，防止单例下次重新初始化
    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);
}

• _dispatch_once_mark_done设置DLOCK_ONCE_DONE

DISPATCH_ALWAYS_INLINE
static inline uintptr_t
_dispatch_once_mark_done(dispatch_once_gate_t dgo)
{
    return os_atomic_xchg(&dgo->dgo_once, DLOCK_ONCE_DONE, release);
}

• 查看_dispatch_once_wait，等待检测DLOCK_ONCE_DONE

void
_dispatch_once_wait(dispatch_once_gate_t dgo)
{
    dispatch_lock self = _dispatch_lock_value_for_self();
    uintptr_t old_v, new_v;
#if HAVE_UL_UNFAIR_LOCK || HAVE_FUTEX
    dispatch_lock *lock = &dgo->dgo_gate.dgl_lock;
#endif
    uint32_t timeout = 1;
...... //省略代码

异步函数分析下

继续上面异步函数分析，上面我们探索到_dispatch_root_queues_init -> _dispatch_root_queues_init_once方法

static void
_dispatch_root_queues_init_once(void *context DISPATCH_UNUSED)
{
    _dispatch_fork_becomes_unsafe();
#if DISPATCH_USE_INTERNAL_WORKQUEUE
    size_t i;
    for (i = 0; i < DISPATCH_ROOT_QUEUE_COUNT; i++) {
        _dispatch_root_queue_init_pthread_pool(&_dispatch_root_queues[i], 0,
                _dispatch_root_queues[i].dq_priority);
    }
#else
    int wq_supported = _pthread_workqueue_supported();
    int r = ENOTSUP;

    if (!(wq_supported & WORKQ_FEATURE_MAINTENANCE)) {
        DISPATCH_INTERNAL_CRASH(wq_supported,
                "QoS Maintenance support required");
    }

#if DISPATCH_USE_KEVENT_SETUP
    struct pthread_workqueue_config cfg = {
        .version = PTHREAD_WORKQUEUE_CONFIG_VERSION,
        .flags = 0,
        .workq_cb = 0,
        .kevent_cb = 0,
        .workloop_cb = 0,
        .queue_serialno_offs = dispatch_queue_offsets.dqo_serialnum,
#if PTHREAD_WORKQUEUE_CONFIG_VERSION >= 2
        .queue_label_offs = dispatch_queue_offsets.dqo_label,
#endif
    };
#endif

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunreachable-code"
    if (unlikely(!_dispatch_kevent_workqueue_enabled)) {
#if DISPATCH_USE_KEVENT_SETUP
        // 执行_dispatch_worker_thread2
        cfg.workq_cb = _dispatch_worker_thread2;
        r = pthread_workqueue_setup(&cfg, sizeof(cfg));
#else
        r = _pthread_workqueue_init(_dispatch_worker_thread2,
                offsetof(struct dispatch_queue_s, dq_serialnum), 0);
#endif // DISPATCH_USE_KEVENT_SETUP
#if DISPATCH_USE_KEVENT_WORKLOOP
    } else if (wq_supported & WORKQ_FEATURE_WORKLOOP) {
#if DISPATCH_USE_KEVENT_SETUP
        // 执行_dispatch_worker_thread2
        cfg.workq_cb = _dispatch_worker_thread2;
        cfg.kevent_cb = (pthread_workqueue_function_kevent_t) _dispatch_kevent_worker_thread;
        cfg.workloop_cb = (pthread_workqueue_function_workloop_t) _dispatch_workloop_worker_thread;
r = pthread_workqueue_setup(&cfg, sizeof(cfg));
#else
        // 执行_dispatch_worker_thread2
        r = _pthread_workqueue_init_with_workloop(_dispatch_worker_thread2,
                (pthread_workqueue_function_kevent_t)
                _dispatch_kevent_worker_thread,
                (pthread_workqueue_function_workloop_t)
                _dispatch_workloop_worker_thread,
                offsetof(struct dispatch_queue_s, dq_serialnum), 0);
......

image.png

让_dispatch_root_queues_init_once执行一次的任务包装在 _dispatch_worker_thread2里面，其实包装在pthread中API中，GCD是封装了pthread。
这里的_pthread_workqueue_init_with_workloop工作循环调起，并不是及时调用的，而是受我们当前OS管控。

• 回到_dispatch_root_queue_poke_slow方法

DISPATCH_NOINLINE
static void
_dispatch_root_queue_poke_slow(dispatch_queue_global_t dq, int n, int floor)
{
        //默认传1 全局并发队列创建1个线程
    int remaining = n;
#if !defined(_WIN32)
    int r = ENOSYS;
#endif

    _dispatch_root_queues_init();
    _dispatch_debug_root_queue(dq, __func__);
    //runtime相关处理
    _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)
#endif
    {
        //全局GLOBAL队列 创建线程
        _dispatch_root_queue_debug("requesting new worker thread for global "
                "queue: %p", dq);
        // 创造线程执行
        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

...... //省略代码
    
    //如果是普通的 进入do while循环
    //remaining空余的数量
    int can_request, t_count;
    // seq_cst with atomic store to tail <rdar://problem/16932833>
    t_count = os_atomic_load2o(dq, dgq_thread_pool_size, ordered);
    do {
        can_request = t_count < floor ? 0 : t_count - floor;
                //大于抛异常
        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;
        }
                //变0抛异常 
        if (remaining == 0) {
            _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));
    //dgq_thread_pool_size 标记为1。

...... //省略代码

    do {
        _dispatch_retain(dq); // released in _dispatch_worker_thread
#if DISPATCH_DEBUG
        unsigned dwStackSize = 0;
#else           
        //到底开多大 
                unsigned dwStackSize = 64 * 1024;
#endif
...... //省略代码

• dgq_thread_pool_size标记为1

struct dispatch_queue_global_s _dispatch_mgr_root_queue = {
    DISPATCH_GLOBAL_OBJECT_HEADER(queue_global),
    .dq_state = DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE,
    .do_ctxt = &_dispatch_mgr_root_queue_pthread_context,
    .dq_label = "com.apple.root.libdispatch-manager",
    .dq_atomic_flags = DQF_WIDTH(DISPATCH_QUEUE_WIDTH_POOL),
    .dq_priority = DISPATCH_PRIORITY_FLAG_MANAGER |
            DISPATCH_PRIORITY_SATURATED_OVERRIDE,
    .dq_serialnum = 3,
    .dgq_thread_pool_size = 1,
};

• DISPATCH_QUEUE_WIDTH_POOL

#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; })

• 全局队列比并发队列大1
  

  image.png

• 通过os_atomic_inc2o自增加++

(void)os_atomic_inc2o(dq, dgq_thread_pool_size, release);

• DISPATCH_WORKQ_MAX_PTHREAD_COUNT线程池最大数量

#ifndef DISPATCH_WORKQ_MAX_PTHREAD_COUNT
#define DISPATCH_WORKQ_MAX_PTHREAD_COUNT 255
#endif

unsigned dwStackSize = 64 * 1024;
最大开辟线程数量1GB = 1024*1024/16kb = 1024*64

线程编程指南

image.png

异步函数调用流程_dispatch_worker_thread2 ->_dispatch_root_queue_drain->_dispatch_async_redirect_invoke->_dispatch_continuation_pop->_dispatch_client_callout->_dispatch_call_block_and_release