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/*
* $Id$
*
* Copyright (C) 2007 iptelorg GmbH
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* futex based lock (mutex) implementation (linux 2.6+ only)
* based on Ulrich Drepper implementation in "Futexes Are Tricky"
* (http://people.redhat.com/drepper/futex.pdf)
*
* Implements:
* void futex_get(futex_lock_t* lock); - mutex lock
* void futex_release(futex_lock_t* lock); - unlock
* int futex_try(futex_lock_t* lock); - tries to get lock, returns 0
* on success and !=0 on failure
* (1 or 2)
*
* Config defines:
*/
/*
* History:
* --------
* 2007-05-13 created by andrei
* 2007-06-12 added ADAPTIVE_WAIT busy waiting (andrei)
*/
#ifndef _futexlock_h
#define _futexlock_h
#include "atomic/atomic_common.h"
#include "atomic/atomic_native.h"
#ifdef HAVE_ASM_INLINE_ATOMIC_OPS
#define HAVE_FUTEX
#include <sys/types.h> /* hack to workaround some type conflicts
between linux-libc-dev andlibc headers
in recent (6.08.2008) x86_64 debian sid
installations */
/* hack to work with old linux/futex.h versions, that depend on sched.h in
__KERNEL__ mode (futex.h < 2.6.20) */
#include <linux/types.h>
typedef __u32 u32;
struct task_struct;
/* end of the hack */
/* another hack this time for OpenSuse 10.2:
futex.h uses a __user attribute, which is defined in linux/compiler.h
However linux/compiler.h is not part of the kernel headers package in
most distributions. Instead they ship a modified linux/futex.h that does
not include <linux/compile.h> and does not user __user.
*/
#ifndef __user
#define __user
#endif /* __user__*/
/* end of hack */
#include <linux/futex.h>
#include <sys/syscall.h>
#include <unistd.h>
#include "compiler_opt.h"
/* either syscall directly or #include <sys/linux/syscall.h> and use
* sys_futex directly */
#define sys_futex(addr, op, val, timeout, addr2, val3) \
syscall(__NR_futex , (addr), (op), (val), (timeout), (addr2), (val3))
typedef atomic_t futex_lock_t;
/* the mutex has 3 states: 0 - free/unlocked and nobody waiting
* 1 - locked and nobody waiting for it
* 2 - locked w/ 0 or more waiting processes/threads
*/
inline static futex_lock_t* futex_init(futex_lock_t* lock)
{
atomic_set(lock, 0);
return lock;
}
inline static void futex_get(futex_lock_t* lock)
{
int v;
#ifdef ADAPTIVE_WAIT
register int i=ADAPTIVE_WAIT_LOOPS;
retry:
#endif
v=atomic_cmpxchg(lock, 0, 1); /* lock if 0 */
if (likely(v==0)){ /* optimize for the uncontended case */
/* success */
membar_enter_lock();
return;
}else if (unlikely(v==2)){ /* if contended, optimize for the one waiter
case */
/* waiting processes/threads => add ourselves to the queue */
do{
sys_futex(&(lock)->val, FUTEX_WAIT, 2, 0, 0, 0);
v=atomic_get_and_set(lock, 2);
}while(v);
}else{
/* v==1 */
#ifdef ADAPTIVE_WAIT
if (i>0){
i--;
goto retry;
}
#endif
v=atomic_get_and_set(lock, 2);
while(v){
sys_futex(&(lock)->val, FUTEX_WAIT, 2, 0, 0, 0);
v=atomic_get_and_set(lock, 2);
}
}
membar_enter_lock();
}
inline static void futex_release(futex_lock_t* lock)
{
int v;
membar_leave_lock();
v=atomic_get_and_set(lock, 0);
if (unlikely(v==2)){ /* optimize for the uncontended case */
sys_futex(&(lock)->val, FUTEX_WAKE, 1, 0, 0, 0);
}
}
static inline int futex_try(futex_lock_t* lock)
{
int c;
c=atomic_cmpxchg(lock, 0, 1);
if (likely(c))
membar_enter_lock();
return c;
}
#else /*HAVE_ASM_INLINE_ATOMIC_OPS*/
#undef USE_FUTEX
#endif /*HAVE_ASM_INLINE_ATOMIC_OPS*/
#endif /* _futexlocks_h*/
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