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mutex.h

/*
 * 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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