include/asm-generic/mutex-dec.h

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