| // -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*- |
| /* Copyright (c) 2006, Google Inc. |
| * All rights reserved. |
| * |
| * Redistribution and use in source and binary forms, with or without |
| * modification, are permitted provided that the following conditions are |
| * met: |
| * |
| * * Redistributions of source code must retain the above copyright |
| * notice, this list of conditions and the following disclaimer. |
| * * Redistributions in binary form must reproduce the above |
| * copyright notice, this list of conditions and the following disclaimer |
| * in the documentation and/or other materials provided with the |
| * distribution. |
| * * Neither the name of Google Inc. nor the names of its |
| * contributors may be used to endorse or promote products derived from |
| * this software without specific prior written permission. |
| * |
| * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS |
| * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT |
| * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR |
| * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT |
| * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
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| * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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| * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE |
| * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| * |
| * --- |
| * Author: Sanjay Ghemawat |
| */ |
| |
| // SpinLock is async signal safe. |
| // If used within a signal handler, all lock holders |
| // should block the signal even outside the signal handler. |
| |
| #ifndef BASE_SPINLOCK_H_ |
| #define BASE_SPINLOCK_H_ |
| |
| #include <config.h> |
| |
| #include <atomic> |
| #include <type_traits> |
| |
| #include "base/basictypes.h" |
| #include "base/dynamic_annotations.h" |
| #include "base/static_storage.h" |
| #include "base/thread_annotations.h" |
| |
| class LOCKABLE SpinLock { |
| public: |
| constexpr SpinLock() : lockword_(kSpinLockFree) { } |
| |
| // Acquire this SpinLock. |
| void Lock() EXCLUSIVE_LOCK_FUNCTION() { |
| int old = kSpinLockFree; |
| if (!lockword_.compare_exchange_weak(old, kSpinLockHeld, std::memory_order_acquire)) { |
| SlowLock(); |
| } |
| } |
| |
| // Try to acquire this SpinLock without blocking and return true if the |
| // acquisition was successful. If the lock was not acquired, false is |
| // returned. If this SpinLock is free at the time of the call, TryLock |
| // will return true with high probability. |
| bool TryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true) { |
| int old = kSpinLockFree; |
| return lockword_.compare_exchange_weak(old, kSpinLockHeld); |
| } |
| |
| // Release this SpinLock, which must be held by the calling thread. |
| void Unlock() UNLOCK_FUNCTION() { |
| int prev_value = lockword_.exchange(kSpinLockFree, std::memory_order_release); |
| if (prev_value != kSpinLockHeld) { |
| // Speed the wakeup of any waiter. |
| SlowUnlock(); |
| } |
| } |
| |
| // Determine if the lock is held. When the lock is held by the invoking |
| // thread, true will always be returned. Intended to be used as |
| // CHECK(lock.IsHeld()). |
| bool IsHeld() const { |
| return lockword_.load(std::memory_order_relaxed) != kSpinLockFree; |
| } |
| |
| private: |
| enum { kSpinLockFree = 0 }; |
| enum { kSpinLockHeld = 1 }; |
| enum { kSpinLockSleeper = 2 }; |
| |
| std::atomic<int> lockword_; |
| |
| void SlowLock(); |
| void SlowUnlock(); |
| int SpinLoop(); |
| |
| DISALLOW_COPY_AND_ASSIGN(SpinLock); |
| }; |
| |
| // Corresponding locker object that arranges to acquire a spinlock for |
| // the duration of a C++ scope. |
| class SCOPED_LOCKABLE SpinLockHolder { |
| private: |
| SpinLock* lock_; |
| public: |
| explicit SpinLockHolder(SpinLock* l) EXCLUSIVE_LOCK_FUNCTION(l) |
| : lock_(l) { |
| l->Lock(); |
| } |
| SpinLockHolder(const SpinLockHolder&) = delete; |
| ~SpinLockHolder() UNLOCK_FUNCTION() { |
| lock_->Unlock(); |
| } |
| }; |
| // Catch bug where variable name is omitted, e.g. SpinLockHolder (&lock); |
| #define SpinLockHolder(x) static_assert(0) |
| |
| namespace tcmalloc { |
| |
| class TrivialOnce { |
| public: |
| template <typename Body> |
| bool RunOnce(Body body) { |
| auto done_atomic = reinterpret_cast<std::atomic<int>*>(&done_flag_); |
| if (done_atomic->load(std::memory_order_acquire) == 1) { |
| return false; |
| } |
| |
| SpinLockHolder h(lock_storage_.get()); |
| |
| if (done_atomic->load(std::memory_order_relaxed) == 1) { |
| // barrier provided by lock |
| return false; |
| } |
| body(); |
| done_atomic->store(1, std::memory_order_release); |
| return true; |
| } |
| |
| private: |
| int done_flag_; |
| StaticStorage<SpinLock> lock_storage_; |
| }; |
| |
| static_assert(std::is_trivial<TrivialOnce>::value == true, ""); |
| |
| } // namespace tcmalloc |
| |
| |
| #endif // BASE_SPINLOCK_H_ |