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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 17:32:43 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 17:32:43 +0000 |
commit | 6bf0a5cb5034a7e684dcc3500e841785237ce2dd (patch) | |
tree | a68f146d7fa01f0134297619fbe7e33db084e0aa /security/sandbox/chromium/base/synchronization/condition_variable.h | |
parent | Initial commit. (diff) | |
download | thunderbird-6bf0a5cb5034a7e684dcc3500e841785237ce2dd.tar.xz thunderbird-6bf0a5cb5034a7e684dcc3500e841785237ce2dd.zip |
Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'security/sandbox/chromium/base/synchronization/condition_variable.h')
-rw-r--r-- | security/sandbox/chromium/base/synchronization/condition_variable.h | 135 |
1 files changed, 135 insertions, 0 deletions
diff --git a/security/sandbox/chromium/base/synchronization/condition_variable.h b/security/sandbox/chromium/base/synchronization/condition_variable.h new file mode 100644 index 0000000000..d92b738081 --- /dev/null +++ b/security/sandbox/chromium/base/synchronization/condition_variable.h @@ -0,0 +1,135 @@ +// 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. + +#ifndef BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ +#define BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ + +#if defined(OS_POSIX) || defined(OS_FUCHSIA) +#include <pthread.h> +#endif + +#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_WIN) +#include "base/win/windows_types.h" +#endif + +namespace base { + +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. The wait functions are + // susceptible to spurious wakeups. (See usage note 1 for more details.) + void Wait(); + void TimedWait(const TimeDelta& max_time); + + // Broadcast() revives all waiting threads. (See usage note 2 for more + // details.) + void Broadcast(); + // Signal() revives one waiting thread. + void Signal(); + + // Declares that this ConditionVariable will only ever be used by a thread + // that is idle at the bottom of its stack and waiting for work (in + // particular, it is not synchronously waiting on this ConditionVariable + // before resuming ongoing work). This is useful to avoid telling + // base-internals that this thread is "blocked" when it's merely idle and + // ready to do work. As such, this is only expected to be used by thread and + // thread pool impls. + void declare_only_used_while_idle() { waiting_is_blocking_ = false; } + + private: + +#if defined(OS_WIN) + CHROME_CONDITION_VARIABLE cv_; + CHROME_SRWLOCK* const srwlock_; +#elif defined(OS_POSIX) || defined(OS_FUCHSIA) + pthread_cond_t condition_; + pthread_mutex_t* user_mutex_; +#endif + +#if DCHECK_IS_ON() + base::Lock* const user_lock_; // Needed to adjust shadow lock state on wait. +#endif + + // Whether a thread invoking Wait() on this ConditionalVariable should be + // considered blocked as opposed to idle (and potentially replaced if part of + // a pool). + bool waiting_is_blocking_ = true; + + DISALLOW_COPY_AND_ASSIGN(ConditionVariable); +}; + +} // namespace base + +#endif // BASE_SYNCHRONIZATION_CONDITION_VARIABLE_H_ |