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/*
* include/asm-arm/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.
*/
#define __mutex_fastpath_lock(count, fail_fn) \
do { \
int __ex_flag, __res; \
\
typecheck(atomic_t *, count); \
typecheck_fn(fastcall void (*)(atomic_t *), fail_fn); \
\
__asm__ ( \
"ldrex %0, [%2] \n" \
"sub %0, %0, #1 \n" \
"strex %1, %0, [%2] \n" \
\
: "=&r" (__res), "=&r" (__ex_flag) \
: "r" (&(count)->counter) \
: "cc","memory" ); \
\
if (unlikely(__res || __ex_flag)) \
fail_fn(count); \
} while (0)
#define __mutex_fastpath_lock_retval(count, fail_fn) \
({ \
int __ex_flag, __res; \
\
typecheck(atomic_t *, count); \
typecheck_fn(fastcall int (*)(atomic_t *), fail_fn); \
\
__asm__ ( \
"ldrex %0, [%2] \n" \
"sub %0, %0, #1 \n" \
"strex %1, %0, [%2] \n" \
\
: "=&r" (__res), "=&r" (__ex_flag) \
: "r" (&(count)->counter) \
: "cc","memory" ); \
\
__res |= __ex_flag; \
if (unlikely(__res != 0)) \
__res = fail_fn(count); \
__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.
*/
#define __mutex_fastpath_unlock(count, fail_fn) \
do { \
int __ex_flag, __res, __orig; \
\
typecheck(atomic_t *, count); \
typecheck_fn(fastcall void (*)(atomic_t *), fail_fn); \
\
__asm__ ( \
"ldrex %0, [%3] \n" \
"add %1, %0, #1 \n" \
"strex %2, %1, [%3] \n" \
\
: "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag) \
: "r" (&(count)->counter) \
: "cc","memory" ); \
\
if (unlikely(__orig || __ex_flag)) \
fail_fn(count); \
} while (0)
/*
* 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"
"subs %1, %0, #1 \n"
"strexeq %2, %1, [%3] \n"
"movlt %0, #0 \n"
"cmpeq %2, #0 \n"
"bgt 1b \n"
: "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag)
: "r" (&count->counter)
: "cc", "memory" );
return __orig;
}
#endif
#endif
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