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// spin_lock.cpp
/**
* Copyright (C) 2010 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include "pch.h" // todo eliminate this include
#include <time.h>
#include "spin_lock.h"
namespace mongo {
SpinLock::~SpinLock() {
#if defined(_WIN32)
DeleteCriticalSection(&_cs);
#elif defined(__USE_XOPEN2K)
pthread_spin_destroy(&_lock);
#endif
}
SpinLock::SpinLock()
#if defined(_WIN32)
{ InitializeCriticalSectionAndSpinCount(&_cs, 4000); }
#elif defined(__USE_XOPEN2K)
{ pthread_spin_init( &_lock , 0 ); }
#elif defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
: _locked( false ) { }
#else
: _mutex( "SpinLock" ) { }
#endif
#if defined(__USE_XOPEN2K)
NOINLINE_DECL void SpinLock::_lk() {
/**
* this is designed to perform close to the default spin lock
* the reason for the mild insanity is to prevent horrible performance
* when contention spikes
* it allows spinlocks to be used in many more places
* which is good because even with this change they are about 8x faster on linux
*/
for ( int i=0; i<1000; i++ ) {
if ( pthread_spin_trylock( &_lock ) == 0 )
return;
asm volatile ( "pause" ) ; // maybe trylock does this; just in case.
}
for ( int i=0; i<1000; i++ ) {
if ( pthread_spin_trylock( &_lock ) == 0 )
return;
pthread_yield();
}
struct timespec t;
t.tv_sec = 0;
t.tv_nsec = 5000000;
while ( pthread_spin_trylock( &_lock ) != 0 ) {
nanosleep(&t, NULL);
}
}
#elif defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
void SpinLock::lock() {
// fast path
if (!_locked && !__sync_lock_test_and_set(&_locked, true)) {
return;
}
// wait for lock
int wait = 1000;
while ((wait-- > 0) && (_locked)) {
asm volatile ( "pause" ) ;
}
// if failed to grab lock, sleep
struct timespec t;
t.tv_sec = 0;
t.tv_nsec = 5000000;
while (__sync_lock_test_and_set(&_locked, true)) {
nanosleep(&t, NULL);
}
}
#endif
bool SpinLock::isfast() {
#if defined(_WIN32) || defined(__USE_XOPEN2K) || defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
return true;
#else
return false;
#endif
}
} // namespace mongo
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