From 6bf0a5cb5034a7e684dcc3500e841785237ce2dd Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 19:32:43 +0200 Subject: Adding upstream version 1:115.7.0. Signed-off-by: Daniel Baumann --- ipc/chromium/src/base/condition_variable.h | 117 +++++++++++++++++++++++++++++ 1 file changed, 117 insertions(+) create mode 100644 ipc/chromium/src/base/condition_variable.h (limited to 'ipc/chromium/src/base/condition_variable.h') diff --git a/ipc/chromium/src/base/condition_variable.h b/ipc/chromium/src/base/condition_variable.h new file mode 100644 index 0000000000..864479ec28 --- /dev/null +++ b/ipc/chromium/src/base/condition_variable.h @@ -0,0 +1,117 @@ +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ +/* vim: set ts=8 sts=2 et sw=2 tw=80: */ +// 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 managed thread pool. +// 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_CONDITION_VARIABLE_H_ +#define BASE_CONDITION_VARIABLE_H_ + +#include "base/basictypes.h" +#include "base/lock.h" +#include "build/build_config.h" + +#if defined(OS_POSIX) +# include +#endif + +#if defined(OS_WIN) +# include +#endif + +namespace base { +class TimeDelta; +} + +class 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. The wait functions are + // susceptible to spurious wakeups. (See usage note 1 for more details.) + void Wait(); + void TimedWait(const base::TimeDelta& max_time); + + // Broadcast() revives all waiting threads. (See usage note 2 for more + // details.) + void Broadcast(); + // Signal() revives one waiting thread. + void Signal(); + + private: +#if defined(OS_WIN) + CONDITION_VARIABLE cv_; + SRWLOCK* const srwlock_; +#elif defined(OS_POSIX) + pthread_cond_t condition_; + pthread_mutex_t* user_mutex_; +#endif + + DISALLOW_COPY_AND_ASSIGN(ConditionVariable); +}; + +#endif // BASE_CONDITION_VARIABLE_H_ -- cgit v1.2.3