/* Copyright (c) 2006, Google Inc.
* All rights reserved.
*
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* 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,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (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
*/
//
// Fast spinlocks (at least on x86, a lock/unlock pair is approximately
// half the cost of a Mutex because the unlock just does a store instead
// of a compare-and-swap which is expensive).
// 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 "base/atomicops.h"
#include "base/basictypes.h"
#include "base/dynamic_annotations.h"
#include "base/thread_annotations.h"
class LOCKABLE SpinLock {
public:
SpinLock() : lockword_(kSpinLockFree) { }
// Special constructor for use with static SpinLock objects. E.g.,
//
// static SpinLock lock(base::LINKER_INITIALIZED);
//
// When intialized using this constructor, we depend on the fact
// that the linker has already initialized the memory appropriately.
// A SpinLock constructed like this can be freely used from global
// initializers without worrying about the order in which global
// initializers run.
explicit SpinLock(base::LinkerInitialized /*x*/) {
// Does nothing; lockword_ is already initialized
}
// Acquire this SpinLock.
// TODO(csilvers): uncomment the annotation when we figure out how to
// support this macro with 0 args (see thread_annotations.h)
inline void Lock() /*EXCLUSIVE_LOCK_FUNCTION()*/ {
if (base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
kSpinLockHeld) != kSpinLockFree) {
SlowLock();
}
ANNOTATE_RWLOCK_ACQUIRED(this, 1);
}
// 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.
inline bool TryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true) {
bool res =
(base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
kSpinLockHeld) == kSpinLockFree);
if (res) {
ANNOTATE_RWLOCK_ACQUIRED(this, 1);
}
return res;
}
// Release this SpinLock, which must be held by the calling thread.
// TODO(csilvers): uncomment the annotation when we figure out how to
// support this macro with 0 args (see thread_annotations.h)
inline void Unlock() /*UNLOCK_FUNCTION()*/ {
uint64 wait_cycles =
static_cast<uint64>(base::subtle::NoBarrier_Load(&lockword_));
ANNOTATE_RWLOCK_RELEASED(this, 1);
base::subtle::Release_Store(&lockword_, kSpinLockFree);
if (wait_cycles != kSpinLockHeld) {
// Collect contentionz profile info, and speed the wakeup of any waiter.
// The wait_cycles value indicates how long this thread spent waiting
// for the lock.
SlowUnlock(wait_cycles);
}
}
// 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()).
inline bool IsHeld() const {
return base::subtle::NoBarrier_Load(&lockword_) != kSpinLockFree;
}
static const base::LinkerInitialized LINKER_INITIALIZED; // backwards compat
private:
enum { kSpinLockFree = 0 };
enum { kSpinLockHeld = 1 };
enum { kSpinLockSleeper = 2 };
volatile Atomic32 lockword_;
void SlowLock();
void SlowUnlock(uint64 wait_cycles);
Atomic32 SpinLoop(int64 initial_wait_timestamp, Atomic32* wait_cycles);
inline int32 CalculateWaitCycles(int64 wait_start_time);
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:
inline explicit SpinLockHolder(SpinLock* l) EXCLUSIVE_LOCK_FUNCTION(l)
: lock_(l) {
l->Lock();
}
// TODO(csilvers): uncomment the annotation when we figure out how to
// support this macro with 0 args (see thread_annotations.h)
inline ~SpinLockHolder() /*UNLOCK_FUNCTION()*/ { lock_->Unlock(); }
};
// Catch bug where variable name is omitted, e.g. SpinLockHolder (&lock);
#define SpinLockHolder(x) COMPILE_ASSERT(0, spin_lock_decl_missing_var_name)
#endif // BASE_SPINLOCK_H_