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/*
* arch/arm/include/asm/mutex.h
*
* ARM optimized mutex locking primitives
*
* Please look into asm-generic/mutex-xchg.h for a formal definition.
*/
#ifndef _ASM_MUTEX_H
#define _ASM_MUTEX_H
#if __LINUX_ARM_ARCH__ < 6
/* On pre-ARMv6 hardware the swp based implementation is the most efficient. */
# include <asm-generic/mutex-xchg.h>
#else
/*
* Attempting to lock a mutex on ARMv6+ can be done with a bastardized
* atomic decrement (it is not a reliable atomic decrement but it satisfies
* the defined semantics for our purpose, while being smaller and faster
* than a real atomic decrement or atomic swap. The idea is to attempt
* decrementing the lock value only once. If once decremented it isn't zero,
* or if its store-back fails due to a dispute on the exclusive store, we
* simply bail out immediately through the slow path where the lock will be
* reattempted until it succeeds.
*/
static inline void
__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
int __ex_flag, __res;
__asm__ (
"ldrex %0, [%2] \n\t"
"sub %0, %0, #1 \n\t"
"strex %1, %0, [%2] "
: "=&r" (__res), "=&r" (__ex_flag)
: "r" (&(count)->counter)
: "cc","memory" );
__res |= __ex_flag;
if (unlikely(__res != 0))
fail_fn(count);
}
static inline int
__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
{
int __ex_flag, __res;
__asm__ (
"ldrex %0, [%2] \n\t"
"sub %0, %0, #1 \n\t"
"strex %1, %0, [%2] "
: "=&r" (__res), "=&r" (__ex_flag)
: "r" (&(count)->counter)
: "cc","memory" );
__res |= __ex_flag;
if (unlikely(__res != 0))
__res = fail_fn(count);
return __res;
}
/*
* Same trick is used for the unlock fast path. However the original value,
* rather than the result, is used to test for success in order to have
* better generated assembly.
*/
static inline void
__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
int __ex_flag, __res, __orig;
__asm__ (
"ldrex %0, [%3] \n\t"
"add %1, %0, #1 \n\t"
"strex %2, %1, [%3] "
: "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag)
: "r" (&(count)->counter)
: "cc","memory" );
__orig |= __ex_flag;
if (unlikely(__orig != 0))
fail_fn(count);
}
/*
* If the unlock was done on a contended lock, or if the unlock simply fails
* then the mutex remains locked.
*/
#define __mutex_slowpath_needs_to_unlock() 1
/*
* For __mutex_fastpath_trylock we use another construct which could be
* described as a "single value cmpxchg".
*
* This provides the needed trylock semantics like cmpxchg would, but it is
* lighter and less generic than a true cmpxchg implementation.
*/
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
int __ex_flag, __res, __orig;
__asm__ (
"1: ldrex %0, [%3] \n\t"
"subs %1, %0, #1 \n\t"
"strexeq %2, %1, [%3] \n\t"
"movlt %0, #0 \n\t"
"cmpeq %2, #0 \n\t"
"bgt 1b "
: "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag)
: "r" (&count->counter)
: "cc", "memory" );
return __orig;
}
#endif
#endif
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