WaitableEvent

base/waitable_event_posix.cc

+// Copyright (c) 2006-2008 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 "base/waitable_event.h"
+
+#include "base/condition_variable.h"
+#include "base/lock.h"
+#include "base/message_loop.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(bool manual_reset, bool initially_signaled)
+    : signaled_(false),
+      manual_reset_(manual_reset) {
+  DCHECK(!initially_signaled) << "Not implemented";
+}
+
+WaitableEvent::~WaitableEvent() {
+  DCHECK(waiters_.empty()) << "Deleting WaitableEvent with listeners!";
+}
+
+void WaitableEvent::Reset() {
+  AutoLock locked(lock_);
+  signaled_ = false;
+}
+
+void WaitableEvent::Signal() {
+  AutoLock locked(lock_);
+
+  if (signaled_)
+    return;
+
+  if (manual_reset_) {
+    SignalAll();
+    signaled_ = true;
+  } else {
+    // In the case of auto reset, if no waiters were woken, we remain
+    // signaled.
+    if (!SignalOne())
+      signaled_ = true;
+  }
+}
+
+bool WaitableEvent::IsSignaled() {
+  AutoLock locked(lock_);
+
+  const bool result = signaled_;
+  if (result && !manual_reset_)
+    signaled_ = false;
+  return result;
+}
+
+// -----------------------------------------------------------------------------
+// Synchronous waits
+
+// -----------------------------------------------------------------------------
+// This is an 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(ConditionVariable* cv, Lock* lock)
+      : fired_(false),
+        cv_(cv),
+        lock_(lock),
+        signaling_event_(NULL) { }
+
+  bool Fire(WaitableEvent *signaling_event) {
+    lock_->Acquire();
+      const bool previous_value = fired_;
+      fired_ = true;
+      if (!previous_value)
+        signaling_event_ = signaling_event;
+    lock_->Release();
+
+    if (previous_value)
+      return false;
+
+    cv_->Broadcast();
+
+    // SyncWaiters are stack allocated on the stack of the blocking thread.
+    return true;
+  }
+
+  WaitableEvent* signaled_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) {
+    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;
+  }
+
+ private:
+  bool fired_;
+  ConditionVariable *const cv_;
+  Lock *const lock_;
+  WaitableEvent* signaling_event_;  // The WaitableEvent which woke us
+};
+
+bool WaitableEvent::TimedWait(const TimeDelta& max_time) {
+  const Time end_time(Time::Now() + max_time);
+
+  lock_.Acquire();
+    if (signaled_) {
+      if (!manual_reset_) {
+        // In this case we were signaled when we had no waiters. Now that
+        // someone has waited upon us, we can automatically reset.
+        signaled_ = false;
+      }
+
+      lock_.Release();
+      return true;
+    }
+
+    Lock lock;
+    lock.Acquire();
+    ConditionVariable cv(&lock);
+    SyncWaiter sw(&cv, &lock);
+
+    Enqueue(&sw);
+  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.
+
+  for (;;) {
+    if (sw.fired()) {
+      lock.Release();
+      return true;
+    }
+
+    if (max_time.ToInternalValue() < 0) {
+      cv.Wait();
+    } else {
+      const Time current_time(Time::Now());
+      if (current_time >= end_time) {
+        // We can't acquire @lock_ before releasing @lock (because of locking
+        // order), however, inbetween 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();
+        lock.Release();
+
+        lock_.Acquire();
+          Dequeue(&sw, &sw);
+        lock_.Release();
+        return false;
+      }
+      const TimeDelta max_wait(end_time - current_time);
+
+      cv.TimedWait(max_wait);
+    }
+  }
+}
+
+bool WaitableEvent::Wait() {
+  return TimedWait(TimeDelta::FromSeconds(-1));
+}
+
+// -----------------------------------------------------------------------------
+
+
+// -----------------------------------------------------------------------------
+// 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
+size_t WaitableEvent::WaitMany(WaitableEvent** raw_waitables,
+                               size_t count) {
+  DCHECK(count) << "Cannot wait on no events";
+
+  // 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);
+  }
+
+  Lock lock;
+  ConditionVariable cv(&lock);
+  SyncWaiter sw(&cv, &lock);
+
+  const size_t r = EnqueueMany(&waitables[0], count, &sw);
+  if (r) {
+    // One of the events is already signaled. The SyncWaiter has not been
+    // enqueued anywhere. EnqueueMany returns the count of remaining waitables
+    // when the signaled one was seen, so the index of the signaled event is
+    // @count - @r.
+    return waitables[count - r].second;
+  }
+
+  // At this point, we hold the locks on all the WaitableEvents and we have
+  // enqueued our waiter in them all.
+  lock.Acquire();
+    // Release the WaitableEvent locks in the reverse order
+    for (size_t i = 0; i < count; ++i) {
+      waitables[count - (1 + i)].first->lock_.Release();
+    }
+
+    for (;;) {
+      if (sw.fired())
+        break;
+
+      cv.Wait();
+    }
+  lock.Release();
+
+  // The address of the WaitableEvent which fired is stored in the SyncWaiter.
+  WaitableEvent *const signaled_event = sw.signaled_event();
+  // This will store the index of the raw_waitables which fired.
+  size_t signaled_index;
+
+  // 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]->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]->Dequeue(&sw, &sw);
+      raw_waitables[i]->lock_.Release();
+    } else {
+      signaled_index = i;
+    }
+  }
+
+  return signaled_index;
+}
+
+// -----------------------------------------------------------------------------
+// If return value == 0:
+//   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 first WaitableEvent
+//   which was signaled, from the end of the array.
+// -----------------------------------------------------------------------------
+// static
+unsigned WaitableEvent::EnqueueMany
+    (std::pair<WaitableEvent*, unsigned>* waitables,
+     unsigned count, Waiter* waiter) {
+  if (!count)
+    return 0;
+
+  waitables[0].first->lock_.Acquire();
+    if (waitables[0].first->signaled_) {
+      if (!waitables[0].first->manual_reset_)
+        waitables[0].first->signaled_ = false;
+      waitables[0].first->lock_.Release();
+      return count;
+    }
+
+    const unsigned r = EnqueueMany(waitables + 1, count - 1, waiter);
+    if (r) {
+      waitables[0].first->lock_.Release();
+    } else {
+      waitables[0].first->Enqueue(waiter);
+    }
+
+    return r;
+}
+
+// -----------------------------------------------------------------------------
+
+
+// -----------------------------------------------------------------------------
+// Private functions...
+
+// -----------------------------------------------------------------------------
+// Wake all waiting waiters. Called with lock held.
+// -----------------------------------------------------------------------------
+bool WaitableEvent::SignalAll() {
+  bool signaled_at_least_one = false;
+
+  for (std::list<Waiter*>::iterator
+       i = waiters_.begin(); i != waiters_.end(); ++i) {
+    if ((*i)->Fire(this))
+      signaled_at_least_one = true;
+  }
+
+  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 (waiters_.empty())
+      return false;
+
+    const bool r = (*waiters_.begin())->Fire(this);
+    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) {
+  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::Dequeue(Waiter* waiter, void* tag) {
+  for (std::list<Waiter*>::iterator
+       i = waiters_.begin(); i != waiters_.end(); ++i) {
+    if (*i == waiter && (*i)->Compare(tag)) {
+      waiters_.erase(i);
+      return true;
+    }
+  }
+
+  return false;
+}
+
+// -----------------------------------------------------------------------------
+
+}  // namespace base