summaryrefslogtreecommitdiffstats
path: root/ipc/chromium/src/base/condition_variable.h
diff options
context:
space:
mode:
authorMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
committerMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
commit5f8de423f190bbb79a62f804151bc24824fa32d8 (patch)
tree10027f336435511475e392454359edea8e25895d /ipc/chromium/src/base/condition_variable.h
parent49ee0794b5d912db1f95dce6eb52d781dc210db5 (diff)
downloadUXP-5f8de423f190bbb79a62f804151bc24824fa32d8.tar
UXP-5f8de423f190bbb79a62f804151bc24824fa32d8.tar.gz
UXP-5f8de423f190bbb79a62f804151bc24824fa32d8.tar.lz
UXP-5f8de423f190bbb79a62f804151bc24824fa32d8.tar.xz
UXP-5f8de423f190bbb79a62f804151bc24824fa32d8.zip
Add m-esr52 at 52.6.0
Diffstat (limited to 'ipc/chromium/src/base/condition_variable.h')
-rw-r--r--ipc/chromium/src/base/condition_variable.h176
1 files changed, 176 insertions, 0 deletions
diff --git a/ipc/chromium/src/base/condition_variable.h b/ipc/chromium/src/base/condition_variable.h
new file mode 100644
index 000000000..4c95dc0bc
--- /dev/null
+++ b/ipc/chromium/src/base/condition_variable.h
@@ -0,0 +1,176 @@
+/* -*- 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) 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.
+
+// 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_CONDITION_VARIABLE_H_
+#define BASE_CONDITION_VARIABLE_H_
+
+#include "base/lock.h"
+
+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.
+ void Wait();
+ void TimedWait(const base::TimeDelta& max_time);
+
+ // Broadcast() revives all waiting threads.
+ void Broadcast();
+ // Signal() revives one waiting thread.
+ void Signal();
+
+ private:
+
+#if defined(OS_WIN)
+
+ // Define Event class that is used to form circularly linked lists.
+ // The list container is an element with NULL as its handle_ value.
+ // The actual list elements have a non-zero handle_ value.
+ // All calls to methods MUST be done under protection of a lock so that links
+ // can be validated. Without the lock, some links might asynchronously
+ // change, and the assertions would fail (as would list change operations).
+ class Event {
+ public:
+ // Default constructor with no arguments creates a list container.
+ Event();
+ ~Event();
+
+ // InitListElement transitions an instance from a container, to an element.
+ void InitListElement();
+
+ // Methods for use on lists.
+ bool IsEmpty() const;
+ void PushBack(Event* other);
+ Event* PopFront();
+ Event* PopBack();
+
+ // Methods for use on list elements.
+ // Accessor method.
+ HANDLE handle() const;
+ // Pull an element from a list (if it's in one).
+ Event* Extract();
+
+ // Method for use on a list element or on a list.
+ bool IsSingleton() const;
+
+ private:
+ // Provide pre/post conditions to validate correct manipulations.
+ bool ValidateAsDistinct(Event* other) const;
+ bool ValidateAsItem() const;
+ bool ValidateAsList() const;
+ bool ValidateLinks() const;
+
+ HANDLE handle_;
+ Event* next_;
+ Event* prev_;
+ DISALLOW_COPY_AND_ASSIGN(Event);
+ };
+
+ // Note that RUNNING is an unlikely number to have in RAM by accident.
+ // This helps with defensive destructor coding in the face of user error.
+ enum RunState { SHUTDOWN = 0, RUNNING = 64213 };
+
+ // Internal implementation methods supporting Wait().
+ Event* GetEventForWaiting();
+ void RecycleEvent(Event* used_event);
+
+ RunState run_state_;
+
+ // Private critical section for access to member data.
+ Lock internal_lock_;
+
+ // Lock that is acquired before calling Wait().
+ Lock& user_lock_;
+
+ // Events that threads are blocked on.
+ Event waiting_list_;
+
+ // Free list for old events.
+ Event recycling_list_;
+ int recycling_list_size_;
+
+ // The number of allocated, but not yet deleted events.
+ int allocation_counter_;
+
+#elif defined(OS_POSIX)
+
+ pthread_cond_t condition_;
+ pthread_mutex_t* user_mutex_;
+
+#endif
+
+ DISALLOW_COPY_AND_ASSIGN(ConditionVariable);
+};
+
+#endif // BASE_CONDITION_VARIABLE_H_