Repo created

TMessagesProj/jni/voip/webrtc/base/synchronization/waitable_event_posix.cc

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+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <limits>
+#include <vector>
+
+#include "base/debug/activity_tracker.h"
+#include "base/logging.h"
+#include "base/optional.h"
+#include "base/synchronization/condition_variable.h"
+#include "base/synchronization/lock.h"
+#include "base/synchronization/waitable_event.h"
+#include "base/threading/scoped_blocking_call.h"
+#include "base/threading/thread_restrictions.h"
+#include "base/time/time.h"
+#include "base/time/time_override.h"
+
+// -----------------------------------------------------------------------------
+// A WaitableEvent on POSIX is implemented as a wait-list. Currently we don't
+// support cross-process events (where one process can signal an event which
+// others are waiting on). Because of this, we can avoid having one thread per
+// listener in several cases.
+//
+// The WaitableEvent maintains a list of waiters, protected by a lock. Each
+// waiter is either an async wait, in which case we have a Task and the
+// MessageLoop to run it on, or a blocking wait, in which case we have the
+// condition variable to signal.
+//
+// Waiting involves grabbing the lock and adding oneself to the wait list. Async
+// waits can be canceled, which means grabbing the lock and removing oneself
+// from the list.
+//
+// Waiting on multiple events is handled by adding a single, synchronous wait to
+// the wait-list of many events. An event passes a pointer to itself when
+// firing a waiter and so we can store that pointer to find out which event
+// triggered.
+// -----------------------------------------------------------------------------
+
+namespace base {
+
+// -----------------------------------------------------------------------------
+// This is just an abstract base class for waking the two types of waiters
+// -----------------------------------------------------------------------------
+WaitableEvent::WaitableEvent(ResetPolicy reset_policy,
+                             InitialState initial_state)
+    : kernel_(new WaitableEventKernel(reset_policy, initial_state)) {}
+
+WaitableEvent::~WaitableEvent() = default;
+
+void WaitableEvent::Reset() {
+  base::AutoLock locked(kernel_->lock_);
+  kernel_->signaled_ = false;
+}
+
+void WaitableEvent::Signal() {
+  base::AutoLock locked(kernel_->lock_);
+
+  if (kernel_->signaled_)
+    return;
+
+  if (kernel_->manual_reset_) {
+    SignalAll();
+    kernel_->signaled_ = true;
+  } else {
+    // In the case of auto reset, if no waiters were woken, we remain
+    // signaled.
+    if (!SignalOne())
+      kernel_->signaled_ = true;
+  }
+}
+
+bool WaitableEvent::IsSignaled() {
+  base::AutoLock locked(kernel_->lock_);
+
+  const bool result = kernel_->signaled_;
+  if (result && !kernel_->manual_reset_)
+    kernel_->signaled_ = false;
+  return result;
+}
+
+// -----------------------------------------------------------------------------
+// Synchronous waits
+
+// -----------------------------------------------------------------------------
+// This is a synchronous waiter. The thread is waiting on the given condition
+// variable and the fired flag in this object.
+// -----------------------------------------------------------------------------
+class SyncWaiter : public WaitableEvent::Waiter {
+ public:
+  SyncWaiter()
+      : fired_(false), signaling_event_(nullptr), lock_(), cv_(&lock_) {}
+
+  bool Fire(WaitableEvent* signaling_event) override {
+    base::AutoLock locked(lock_);
+
+    if (fired_)
+      return false;
+
+    fired_ = true;
+    signaling_event_ = signaling_event;
+
+    cv_.Broadcast();
+
+    // Unlike AsyncWaiter objects, SyncWaiter objects are stack-allocated on
+    // the blocking thread's stack.  There is no |delete this;| in Fire.  The
+    // SyncWaiter object is destroyed when it goes out of scope.
+
+    return true;
+  }
+
+  WaitableEvent* signaling_event() const {
+    return signaling_event_;
+  }
+
+  // ---------------------------------------------------------------------------
+  // These waiters are always stack allocated and don't delete themselves. Thus
+  // there's no problem and the ABA tag is the same as the object pointer.
+  // ---------------------------------------------------------------------------
+  bool Compare(void* tag) override { return this == tag; }
+
+  // ---------------------------------------------------------------------------
+  // Called with lock held.
+  // ---------------------------------------------------------------------------
+  bool fired() const {
+    return fired_;
+  }
+
+  // ---------------------------------------------------------------------------
+  // During a TimedWait, we need a way to make sure that an auto-reset
+  // WaitableEvent doesn't think that this event has been signaled between
+  // unlocking it and removing it from the wait-list. Called with lock held.
+  // ---------------------------------------------------------------------------
+  void Disable() {
+    fired_ = true;
+  }
+
+  base::Lock* lock() {
+    return &lock_;
+  }
+
+  base::ConditionVariable* cv() {
+    return &cv_;
+  }
+
+ private:
+  bool fired_;
+  WaitableEvent* signaling_event_;  // The WaitableEvent which woke us
+  base::Lock lock_;
+  base::ConditionVariable cv_;
+};
+
+void WaitableEvent::Wait() {
+  bool result = TimedWait(TimeDelta::Max());
+  DCHECK(result) << "TimedWait() should never fail with infinite timeout";
+}
+
+bool WaitableEvent::TimedWait(const TimeDelta& wait_delta) {
+  if (wait_delta <= TimeDelta())
+    return IsSignaled();
+
+  // Record the event that this thread is blocking upon (for hang diagnosis) and
+  // consider it blocked for scheduling purposes. Ignore this for non-blocking
+  // WaitableEvents.
+  Optional<debug::ScopedEventWaitActivity> event_activity;
+  Optional<internal::ScopedBlockingCallWithBaseSyncPrimitives>
+      scoped_blocking_call;
+  if (waiting_is_blocking_) {
+    event_activity.emplace(this);
+    scoped_blocking_call.emplace(FROM_HERE, BlockingType::MAY_BLOCK);
+  }
+
+  kernel_->lock_.Acquire();
+  if (kernel_->signaled_) {
+    if (!kernel_->manual_reset_) {
+      // In this case we were signaled when we had no waiters. Now that
+      // someone has waited upon us, we can automatically reset.
+      kernel_->signaled_ = false;
+    }
+
+    kernel_->lock_.Release();
+    return true;
+  }
+
+  SyncWaiter sw;
+  if (!waiting_is_blocking_)
+    sw.cv()->declare_only_used_while_idle();
+  sw.lock()->Acquire();
+
+  Enqueue(&sw);
+  kernel_->lock_.Release();
+  // We are violating locking order here by holding the SyncWaiter lock but not
+  // the WaitableEvent lock. However, this is safe because we don't lock |lock_|
+  // again before unlocking it.
+
+  // TimeTicks takes care of overflow but we special case is_max() nonetheless
+  // to avoid invoking TimeTicksNowIgnoringOverride() unnecessarily (same for
+  // the increment step of the for loop if the condition variable returns
+  // early). Ref: https://crbug.com/910524#c7
+  const TimeTicks end_time =
+      wait_delta.is_max() ? TimeTicks::Max()
+                          : subtle::TimeTicksNowIgnoringOverride() + wait_delta;
+  for (TimeDelta remaining = wait_delta; remaining > TimeDelta() && !sw.fired();
+       remaining = end_time.is_max()
+                       ? TimeDelta::Max()
+                       : end_time - subtle::TimeTicksNowIgnoringOverride()) {
+    if (end_time.is_max())
+      sw.cv()->Wait();
+    else
+      sw.cv()->TimedWait(remaining);
+  }
+
+  // Get the SyncWaiter signaled state before releasing the lock.
+  const bool return_value = sw.fired();
+
+  // We can't acquire |lock_| before releasing the SyncWaiter lock (because of
+  // locking order), however, in between the two a signal could be fired and
+  // |sw| would accept it, however we will still return false, so the signal
+  // would be lost on an auto-reset WaitableEvent. Thus we call Disable which
+  // makes sw::Fire return false.
+  sw.Disable();
+  sw.lock()->Release();
+
+  // This is a bug that has been enshrined in the interface of WaitableEvent
+  // now: |Dequeue| is called even when |sw.fired()| is true, even though it'll
+  // always return false in that case. However, taking the lock ensures that
+  // |Signal| has completed before we return and means that a WaitableEvent can
+  // synchronise its own destruction.
+  kernel_->lock_.Acquire();
+  kernel_->Dequeue(&sw, &sw);
+  kernel_->lock_.Release();
+
+  return return_value;
+}
+
+// -----------------------------------------------------------------------------
+// Synchronous waiting on multiple objects.
+
+static bool  // StrictWeakOrdering
+cmp_fst_addr(const std::pair<WaitableEvent*, unsigned> &a,
+             const std::pair<WaitableEvent*, unsigned> &b) {
+  return a.first < b.first;
+}
+
+// static
+// NO_THREAD_SAFETY_ANALYSIS: Complex control flow.
+size_t WaitableEvent::WaitMany(WaitableEvent** raw_waitables,
+                               size_t count) NO_THREAD_SAFETY_ANALYSIS {
+  DCHECK(count) << "Cannot wait on no events";
+  internal::ScopedBlockingCallWithBaseSyncPrimitives scoped_blocking_call(
+      FROM_HERE, BlockingType::MAY_BLOCK);
+  // Record an event (the first) that this thread is blocking upon.
+  debug::ScopedEventWaitActivity event_activity(raw_waitables[0]);
+
+  // We need to acquire the locks in a globally consistent order. Thus we sort
+  // the array of waitables by address. We actually sort a pairs so that we can
+  // map back to the original index values later.
+  std::vector<std::pair<WaitableEvent*, size_t> > waitables;
+  waitables.reserve(count);
+  for (size_t i = 0; i < count; ++i)
+    waitables.push_back(std::make_pair(raw_waitables[i], i));
+
+  DCHECK_EQ(count, waitables.size());
+
+  sort(waitables.begin(), waitables.end(), cmp_fst_addr);
+
+  // The set of waitables must be distinct. Since we have just sorted by
+  // address, we can check this cheaply by comparing pairs of consecutive
+  // elements.
+  for (size_t i = 0; i < waitables.size() - 1; ++i) {
+    DCHECK(waitables[i].first != waitables[i+1].first);
+  }
+
+  SyncWaiter sw;
+
+  const size_t r = EnqueueMany(&waitables[0], count, &sw);
+  if (r < count) {
+    // One of the events is already signaled. The SyncWaiter has not been
+    // enqueued anywhere.
+    return waitables[r].second;
+  }
+
+  // At this point, we hold the locks on all the WaitableEvents and we have
+  // enqueued our waiter in them all.
+  sw.lock()->Acquire();
+    // Release the WaitableEvent locks in the reverse order
+    for (size_t i = 0; i < count; ++i) {
+      waitables[count - (1 + i)].first->kernel_->lock_.Release();
+    }
+
+    for (;;) {
+      if (sw.fired())
+        break;
+
+      sw.cv()->Wait();
+    }
+  sw.lock()->Release();
+
+  // The address of the WaitableEvent which fired is stored in the SyncWaiter.
+  WaitableEvent *const signaled_event = sw.signaling_event();
+  // This will store the index of the raw_waitables which fired.
+  size_t signaled_index = 0;
+
+  // Take the locks of each WaitableEvent in turn (except the signaled one) and
+  // remove our SyncWaiter from the wait-list
+  for (size_t i = 0; i < count; ++i) {
+    if (raw_waitables[i] != signaled_event) {
+      raw_waitables[i]->kernel_->lock_.Acquire();
+        // There's no possible ABA issue with the address of the SyncWaiter here
+        // because it lives on the stack. Thus the tag value is just the pointer
+        // value again.
+        raw_waitables[i]->kernel_->Dequeue(&sw, &sw);
+      raw_waitables[i]->kernel_->lock_.Release();
+    } else {
+      // By taking this lock here we ensure that |Signal| has completed by the
+      // time we return, because |Signal| holds this lock. This matches the
+      // behaviour of |Wait| and |TimedWait|.
+      raw_waitables[i]->kernel_->lock_.Acquire();
+      raw_waitables[i]->kernel_->lock_.Release();
+      signaled_index = i;
+    }
+  }
+
+  return signaled_index;
+}
+
+// -----------------------------------------------------------------------------
+// If return value == count:
+//   The locks of the WaitableEvents have been taken in order and the Waiter has
+//   been enqueued in the wait-list of each. None of the WaitableEvents are
+//   currently signaled
+// else:
+//   None of the WaitableEvent locks are held. The Waiter has not been enqueued
+//   in any of them and the return value is the index of the WaitableEvent which
+//   was signaled with the lowest input index from the original WaitMany call.
+// -----------------------------------------------------------------------------
+// static
+// NO_THREAD_SAFETY_ANALYSIS: Complex control flow.
+size_t WaitableEvent::EnqueueMany(std::pair<WaitableEvent*, size_t>* waitables,
+                                  size_t count,
+                                  Waiter* waiter) NO_THREAD_SAFETY_ANALYSIS {
+  size_t winner = count;
+  size_t winner_index = count;
+  for (size_t i = 0; i < count; ++i) {
+    auto& kernel = waitables[i].first->kernel_;
+    kernel->lock_.Acquire();
+    if (kernel->signaled_ && waitables[i].second < winner) {
+      winner = waitables[i].second;
+      winner_index = i;
+    }
+  }
+
+  // No events signaled. All locks acquired. Enqueue the Waiter on all of them
+  // and return.
+  if (winner == count) {
+    for (size_t i = 0; i < count; ++i)
+      waitables[i].first->Enqueue(waiter);
+    return count;
+  }
+
+  // Unlock in reverse order and possibly clear the chosen winner's signal
+  // before returning its index.
+  for (auto* w = waitables + count - 1; w >= waitables; --w) {
+    auto& kernel = w->first->kernel_;
+    if (w->second == winner) {
+      if (!kernel->manual_reset_)
+        kernel->signaled_ = false;
+    }
+    kernel->lock_.Release();
+  }
+
+  return winner_index;
+}
+
+// -----------------------------------------------------------------------------
+
+
+// -----------------------------------------------------------------------------
+// Private functions...
+
+WaitableEvent::WaitableEventKernel::WaitableEventKernel(
+    ResetPolicy reset_policy,
+    InitialState initial_state)
+    : manual_reset_(reset_policy == ResetPolicy::MANUAL),
+      signaled_(initial_state == InitialState::SIGNALED) {}
+
+WaitableEvent::WaitableEventKernel::~WaitableEventKernel() = default;
+
+// -----------------------------------------------------------------------------
+// Wake all waiting waiters. Called with lock held.
+// -----------------------------------------------------------------------------
+bool WaitableEvent::SignalAll() {
+  bool signaled_at_least_one = false;
+
+  for (auto* i : kernel_->waiters_) {
+    if (i->Fire(this))
+      signaled_at_least_one = true;
+  }
+
+  kernel_->waiters_.clear();
+  return signaled_at_least_one;
+}
+
+// ---------------------------------------------------------------------------
+// Try to wake a single waiter. Return true if one was woken. Called with lock
+// held.
+// ---------------------------------------------------------------------------
+bool WaitableEvent::SignalOne() {
+  for (;;) {
+    if (kernel_->waiters_.empty())
+      return false;
+
+    const bool r = (*kernel_->waiters_.begin())->Fire(this);
+    kernel_->waiters_.pop_front();
+    if (r)
+      return true;
+  }
+}
+
+// -----------------------------------------------------------------------------
+// Add a waiter to the list of those waiting. Called with lock held.
+// -----------------------------------------------------------------------------
+void WaitableEvent::Enqueue(Waiter* waiter) {
+  kernel_->waiters_.push_back(waiter);
+}
+
+// -----------------------------------------------------------------------------
+// Remove a waiter from the list of those waiting. Return true if the waiter was
+// actually removed. Called with lock held.
+// -----------------------------------------------------------------------------
+bool WaitableEvent::WaitableEventKernel::Dequeue(Waiter* waiter, void* tag) {
+  for (auto i = waiters_.begin(); i != waiters_.end(); ++i) {
+    if (*i == waiter && (*i)->Compare(tag)) {
+      waiters_.erase(i);
+      return true;
+    }
+  }
+
+  return false;
+}
+
+// -----------------------------------------------------------------------------
+
+}  // namespace base