root/include/asm-x86/mutex_32.h

INCLUDED FROM

DEFINITIONS

1. __mutex_fastpath_lock_retval
2. __mutex_fastpath_trylock

/*
 * Assembly implementation of the mutex fastpath, based on atomic
 * decrement/increment.
 *
 * started by Ingo Molnar:
 *
 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 */
#ifndef ASM_X86__MUTEX_32_H
#define ASM_X86__MUTEX_32_H

#include <asm/alternative.h>

/**
 *  __mutex_fastpath_lock - try to take the lock by moving the count
 *                          from 1 to a 0 value
 *  @count: pointer of type atomic_t
 *  @fn: function to call if the original value was not 1
 *
 * Change the count from 1 to a value lower than 1, and call <fn> if it
 * wasn't 1 originally. This function MUST leave the value lower than 1
 * even when the "1" assertion wasn't true.
 */
#define __mutex_fastpath_lock(count, fail_fn)                   \
do {                                                            \
        unsigned int dummy;                                     \
                                                                \
        typecheck(atomic_t *, count);                           \
        typecheck_fn(void (*)(atomic_t *), fail_fn);            \
                                                                \
        asm volatile(LOCK_PREFIX "   decl (%%eax)\n"            \
                     "   jns 1f \n"                             \
                     "   call " #fail_fn "\n"                   \
                     "1:\n"                                     \
                     : "=a" (dummy)                             \
                     : "a" (count)                              \
                     : "memory", "ecx", "edx");                 \
} while (0)


/**
 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
 *                                 from 1 to a 0 value
 *  @count: pointer of type atomic_t
 *  @fail_fn: function to call if the original value was not 1
 *
 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
 * or anything the slow path function returns
 */
static inline int __mutex_fastpath_lock_retval(atomic_t *count,
                                               int (*fail_fn)(atomic_t *))
{
        if (unlikely(atomic_dec_return(count) < 0))
                return fail_fn(count);
        else
                return 0;
}

/**
 *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
 *  @count: pointer of type atomic_t
 *  @fail_fn: function to call if the original value was not 0
 *
 * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
 * In the failure case, this function is allowed to either set the value
 * to 1, or to set it to a value lower than 1.
 *
 * If the implementation sets it to a value of lower than 1, the
 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
 * to return 0 otherwise.
 */
#define __mutex_fastpath_unlock(count, fail_fn)                 \
do {                                                            \
        unsigned int dummy;                                     \
                                                                \
        typecheck(atomic_t *, count);                           \
        typecheck_fn(void (*)(atomic_t *), fail_fn);            \
                                                                \
        asm volatile(LOCK_PREFIX "   incl (%%eax)\n"            \
                     "   jg     1f\n"                           \
                     "   call " #fail_fn "\n"                   \
                     "1:\n"                                     \
                     : "=a" (dummy)                             \
                     : "a" (count)                              \
                     : "memory", "ecx", "edx");                 \
} while (0)

#define __mutex_slowpath_needs_to_unlock()      1

/**
 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
 *
 *  @count: pointer of type atomic_t
 *  @fail_fn: fallback function
 *
 * Change the count from 1 to a value lower than 1, and return 0 (failure)
 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
 * Additionally, if the value was < 0 originally, this function must not leave
 * it to 0 on failure.
 */
static inline int __mutex_fastpath_trylock(atomic_t *count,
                                           int (*fail_fn)(atomic_t *))
{
        /*
         * We have two variants here. The cmpxchg based one is the best one
         * because it never induce a false contention state.  It is included
         * here because architectures using the inc/dec algorithms over the
         * xchg ones are much more likely to support cmpxchg natively.
         *
         * If not we fall back to the spinlock based variant - that is
         * just as efficient (and simpler) as a 'destructive' probing of
         * the mutex state would be.
         */
#ifdef __HAVE_ARCH_CMPXCHG
        if (likely(atomic_cmpxchg(count, 1, 0) == 1))
                return 1;
        return 0;
#else
        return fail_fn(count);
#endif
}

#endif /* ASM_X86__MUTEX_32_H */