LXR linux/include/asm-generic/mutex-dec.h

   1/*
   2 * include/asm-generic/mutex-dec.h
   3 *
   4 * Generic implementation of the mutex fastpath, based on atomic
   5 * decrement/increment.
   6 */
   7#ifndef _ASM_GENERIC_MUTEX_DEC_H
   8#define _ASM_GENERIC_MUTEX_DEC_H
   9
  10/**
  11 *  __mutex_fastpath_lock - try to take the lock by moving the count
  12 *                          from 1 to a 0 value
  13 *  @count: pointer of type atomic_t
  14 *  @fail_fn: function to call if the original value was not 1
  15 *
  16 * Change the count from 1 to a value lower than 1, and call <fail_fn> if
  17 * it wasn't 1 originally. This function MUST leave the value lower than
  18 * 1 even when the "1" assertion wasn't true.
  19 */
  20static inline void
  21__mutex_fastpath_lock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
  22{
  23        if (unlikely(atomic_dec_return(count) < 0))
  24                fail_fn(count);
  25        else
  26                smp_mb();
  27}
  28
  29/**
  30 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  31 *                                 from 1 to a 0 value
  32 *  @count: pointer of type atomic_t
  33 *  @fail_fn: function to call if the original value was not 1
  34 *
  35 * Change the count from 1 to a value lower than 1, and call <fail_fn> if
  36 * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  37 * or anything the slow path function returns.
  38 */
  39static inline int
  40__mutex_fastpath_lock_retval(atomic_t *count, fastcall int (*fail_fn)(atomic_t *))
  41{
  42        if (unlikely(atomic_dec_return(count) < 0))
  43                return fail_fn(count);
  44        else {
  45                smp_mb();
  46                return 0;
  47        }
  48}
  49
  50/**
  51 *  __mutex_fastpath_unlock - try to promote the count from 0 to 1
  52 *  @count: pointer of type atomic_t
  53 *  @fail_fn: function to call if the original value was not 0
  54 *
  55 * Try to promote the count from 0 to 1. If it wasn't 0, call <fail_fn>.
  56 * In the failure case, this function is allowed to either set the value to
  57 * 1, or to set it to a value lower than 1.
  58 *
  59 * If the implementation sets it to a value of lower than 1, then the
  60 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
  61 * to return 0 otherwise.
  62 */
  63static inline void
  64__mutex_fastpath_unlock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
  65{
  66        smp_mb();
  67        if (unlikely(atomic_inc_return(count) <= 0))
  68                fail_fn(count);
  69}
  70
  71#define __mutex_slowpath_needs_to_unlock()              1
  72
  73/**
  74 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  75 *
  76 *  @count: pointer of type atomic_t
  77 *  @fail_fn: fallback function
  78 *
  79 * Change the count from 1 to a value lower than 1, and return 0 (failure)
  80 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
  81 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
  82 * Additionally, if the value was < 0 originally, this function must not leave
  83 * it to 0 on failure.
  84 *
  85 * If the architecture has no effective trylock variant, it should call the
  86 * <fail_fn> spinlock-based trylock variant unconditionally.
  87 */
  88static inline int
  89__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  90{
  91        /*
  92         * We have two variants here. The cmpxchg based one is the best one
  93         * because it never induce a false contention state.  It is included
  94         * here because architectures using the inc/dec algorithms over the
  95         * xchg ones are much more likely to support cmpxchg natively.
  96         *
  97         * If not we fall back to the spinlock based variant - that is
  98         * just as efficient (and simpler) as a 'destructive' probing of
  99         * the mutex state would be.
 100         */
 101#ifdef __HAVE_ARCH_CMPXCHG
 102        if (likely(atomic_cmpxchg(count, 1, 0) == 1)) {
 103                smp_mb();
 104                return 1;
 105        }
 106        return 0;
 107#else
 108        return fail_fn(count);
 109#endif
 110}
 111
 112#endif
 113