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41c3e177f8
Differential Revision: https://phabricator.services.mozilla.com/D64358 --HG-- extra : moz-landing-system : lando
84 lines
2.5 KiB
C++
84 lines
2.5 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_StaticMutex_h
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#define mozilla_StaticMutex_h
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#include "mozilla/Atomics.h"
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#include "mozilla/Mutex.h"
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namespace mozilla {
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/**
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* StaticMutex is a Mutex that can (and in fact, must) be used as a
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* global/static variable.
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*
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* The main reason to use StaticMutex as opposed to
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* StaticAutoPtr<OffTheBooksMutex> is that we instantiate the StaticMutex in a
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* thread-safe manner the first time it's used.
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*
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* The same caveats that apply to StaticAutoPtr apply to StaticMutex. In
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* particular, do not use StaticMutex as a stack variable or a class instance
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* variable, because this class relies on the fact that global variablies are
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* initialized to 0 in order to initialize mMutex. It is only safe to use
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* StaticMutex as a global or static variable.
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*/
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class MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS StaticMutex {
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public:
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// In debug builds, check that mMutex is initialized for us as we expect by
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// the compiler. In non-debug builds, don't declare a constructor so that
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// the compiler can see that the constructor is trivial.
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#ifdef DEBUG
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StaticMutex() { MOZ_ASSERT(!mMutex); }
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#endif
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void Lock() { Mutex()->Lock(); }
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void Unlock() { Mutex()->Unlock(); }
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void AssertCurrentThreadOwns() {
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#ifdef DEBUG
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Mutex()->AssertCurrentThreadOwns();
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#endif
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}
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private:
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OffTheBooksMutex* Mutex() {
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if (mMutex) {
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return mMutex;
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}
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OffTheBooksMutex* mutex = new OffTheBooksMutex("StaticMutex");
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if (!mMutex.compareExchange(nullptr, mutex)) {
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delete mutex;
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}
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return mMutex;
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}
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Atomic<OffTheBooksMutex*, SequentiallyConsistent> mMutex;
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// Disallow copy constructor, but only in debug mode. We only define
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// a default constructor in debug mode (see above); if we declared
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// this constructor always, the compiler wouldn't generate a trivial
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// default constructor for us in non-debug mode.
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#ifdef DEBUG
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StaticMutex(StaticMutex& aOther);
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#endif
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// Disallow these operators.
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StaticMutex& operator=(StaticMutex* aRhs);
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static void* operator new(size_t) noexcept(true);
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static void operator delete(void*);
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};
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typedef detail::BaseAutoLock<StaticMutex&> StaticMutexAutoLock;
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typedef detail::BaseAutoUnlock<StaticMutex&> StaticMutexAutoUnlock;
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} // namespace mozilla
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#endif
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