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 static 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  */
  39 static 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  */
  63 static 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  */
  88 static 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