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authorwolfbeast <mcwerewolf@gmail.com>2018-05-03 05:55:15 +0200
committerwolfbeast <mcwerewolf@gmail.com>2018-05-03 05:55:15 +0200
commit43f7a588f96aaf88e7b69441c3b50bc9c7b20df7 (patch)
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Nuke the sandbox
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-// Copyright (c) 2011 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.
-
-// ConditionVariable wraps pthreads condition variable synchronization or, on
-// Windows, simulates it. This functionality is very helpful for having
-// several threads wait for an event, as is common with a thread pool managed
-// by a master. The meaning of such an event in the (worker) thread pool
-// scenario is that additional tasks are now available for processing. It is
-// used in Chrome in the DNS prefetching system to notify worker threads that
-// a queue now has items (tasks) which need to be tended to. A related use
-// would have a pool manager waiting on a ConditionVariable, waiting for a
-// thread in the pool to announce (signal) that there is now more room in a
-// (bounded size) communications queue for the manager to deposit tasks, or,
-// as a second example, that the queue of tasks is completely empty and all
-// workers are waiting.
-//
-// USAGE NOTE 1: spurious signal events are possible with this and
-// most implementations of condition variables. As a result, be
-// *sure* to retest your condition before proceeding. The following
-// is a good example of doing this correctly:
-//
-// while (!work_to_be_done()) Wait(...);
-//
-// In contrast do NOT do the following:
-//
-// if (!work_to_be_done()) Wait(...); // Don't do this.
-//
-// Especially avoid the above if you are relying on some other thread only
-// issuing a signal up *if* there is work-to-do. There can/will
-// be spurious signals. Recheck state on waiting thread before
-// assuming the signal was intentional. Caveat caller ;-).
-//
-// USAGE NOTE 2: Broadcast() frees up all waiting threads at once,
-// which leads to contention for the locks they all held when they
-// called Wait(). This results in POOR performance. A much better
-// approach to getting a lot of threads out of Wait() is to have each
-// thread (upon exiting Wait()) call Signal() to free up another
-// Wait'ing thread. Look at condition_variable_unittest.cc for
-// both examples.
-//
-// Broadcast() can be used nicely during teardown, as it gets the job
-// done, and leaves no sleeping threads... and performance is less
-// critical at that point.
-//
-// The semantics of Broadcast() are carefully crafted so that *all*
-// threads that were waiting when the request was made will indeed
-// get signaled. Some implementations mess up, and don't signal them
-// all, while others allow the wait to be effectively turned off (for
-// a while while waiting threads come around). This implementation
-// appears correct, as it will not "lose" any signals, and will guarantee
-// that all threads get signaled by Broadcast().
-//
-// This implementation offers support for "performance" in its selection of
-// which thread to revive. Performance, in direct contrast with "fairness,"
-// assures that the thread that most recently began to Wait() is selected by
-// Signal to revive. Fairness would (if publicly supported) assure that the
-// thread that has Wait()ed the longest is selected. The default policy
-// may improve performance, as the selected thread may have a greater chance of
-// having some of its stack data in various CPU caches.
-//
-// For a discussion of the many very subtle implementation details, see the FAQ
-// at the end of condition_variable_win.cc.
-
-#ifndef BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_
-#define BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_
-
-#include "base/base_export.h"
-#include "base/logging.h"
-#include "base/macros.h"
-#include "base/synchronization/lock.h"
-#include "build/build_config.h"
-
-#if defined(OS_POSIX)
-#include <pthread.h>
-#endif
-
-namespace base {
-
-class ConditionVarImpl;
-class TimeDelta;
-
-class BASE_EXPORT ConditionVariable {
- public:
- // Construct a cv for use with ONLY one user lock.
- explicit ConditionVariable(Lock* user_lock);
-
- ~ConditionVariable();
-
- // Wait() releases the caller's critical section atomically as it starts to
- // sleep, and the reacquires it when it is signaled.
- void Wait();
- void TimedWait(const TimeDelta& max_time);
-
- // Broadcast() revives all waiting threads.
- void Broadcast();
- // Signal() revives one waiting thread.
- void Signal();
-
- private:
-
-#if defined(OS_WIN)
- ConditionVarImpl* impl_;
-#elif defined(OS_POSIX)
- pthread_cond_t condition_;
- pthread_mutex_t* user_mutex_;
-#if DCHECK_IS_ON()
- base::Lock* user_lock_; // Needed to adjust shadow lock state on wait.
-#endif
-
-#endif
-
- DISALLOW_COPY_AND_ASSIGN(ConditionVariable);
-};
-
-} // namespace base
-
-#endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_