The Tor Browser: ipc/chromium/src/base/waitable_event

ipc/chromium/src/base/waitable_event_posix.cc@6474c204b198 (annotated)

ipc/chromium/src/base/waitable_event_posix.cc

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 // 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)
     : kernel_(new WaitableEventKernel(manual_reset, initially_signaled)) {
 }
 WaitableEvent::~WaitableEvent() {
 }
 void WaitableEvent::Reset() {
   AutoLock locked(kernel_->lock_);
   kernel_->signaled_ = false;
 }
 void WaitableEvent::Signal() {
   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() {
   AutoLock locked(kernel_->lock_);
   const bool result = kernel_->signaled_;
   if (result && !kernel_->manual_reset_)
     kernel_->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 TimeTicks end_time(TimeTicks::Now() + max_time);
   const bool finite_time = max_time.ToInternalValue() >= 0;
   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;
     }
     Lock lock;
     lock.Acquire();
     ConditionVariable cv(&lock);
     SyncWaiter sw(&cv, &lock);
     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.
   for (;;) {
     const TimeTicks current_time(TimeTicks::Now());
     if (sw.fired() || (finite_time && current_time >= end_time)) {
       const bool return_value = sw.fired();
       // 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();
       kernel_->lock_.Acquire();
         kernel_->Dequeue(&sw, &sw);
       kernel_->lock_.Release();
       return return_value;
     }
     if (finite_time) {
       const TimeDelta max_wait(end_time - current_time);
       cv.TimedWait(max_wait);
     } else {
       cv.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->kernel_->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 = 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 {
       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
 size_t WaitableEvent::EnqueueMany
     (std::pair<WaitableEvent*, size_t>* waitables,
      size_t count, Waiter* waiter) {
   if (!count)
     return 0;
   waitables[0].first->kernel_->lock_.Acquire();
     if (waitables[0].first->kernel_->signaled_) {
       if (!waitables[0].first->kernel_->manual_reset_)
         waitables[0].first->kernel_->signaled_ = false;
       waitables[0].first->kernel_->lock_.Release();
       return count;
     }
     const size_t r = EnqueueMany(waitables + 1, count - 1, waiter);
     if (r) {
       waitables[0].first->kernel_->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 = kernel_->waiters_.begin(); i != kernel_->waiters_.end(); ++i) {
     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 (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

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ipc/chromium/src/base/waitable_event_posix.cc@6474c204b198 (annotated)

ipc/chromium/src/base/waitable_event_posix.cc