include/asm-generic/mutex-xchg.h

   1 /*
   2  * include/asm-generic/mutex-xchg.h
   3  *
   4  * Generic implementation of the mutex fastpath, based on xchg().
   5  *
   6  * NOTE: An xchg based implementation might be less optimal than an atomic
   7  *       decrement/increment based implementation. If your architecture
   8  *       has a reasonable atomic dec/inc then you should probably use
   9  *       asm-generic/mutex-dec.h instead, or you could open-code an
  10  *       optimized version in asm/mutex.h.
  11  */
  12 #ifndef _ASM_GENERIC_MUTEX_XCHG_H
  13 #define _ASM_GENERIC_MUTEX_XCHG_H
  14
  15 /**
  16  *  __mutex_fastpath_lock - try to take the lock by moving the count
  17  *                          from 1 to a 0 value
  18  *  @count: pointer of type atomic_t
  19  *  @fail_fn: function to call if the original value was not 1
  20  *
  21  * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  22  * wasn't 1 originally. This function MUST leave the value lower than 1
  23  * even when the "1" assertion wasn't true.
  24  */
  25 static inline void
  26 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
  27 {
  28         if (unlikely(atomic_xchg(count, 0) != 1))
  29                 fail_fn(count);
  30 }
  31
  32 /**
  33  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  34  *                                 from 1 to a 0 value
  35  *  @count: pointer of type atomic_t
  36  *  @fail_fn: function to call if the original value was not 1
  37  *
  38  * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  39  * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  40  * or anything the slow path function returns
  41  */
  42 static inline int
  43 __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
  44 {
  45         if (unlikely(atomic_xchg(count, 0) != 1))
  46                 return fail_fn(count);
  47         return 0;
  48 }
  49
  50 /**
  51  *  __mutex_fastpath_unlock - try to promote the mutex 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 mutex from 0 to 1. if it wasn't 0, call <function>
  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 one.
  58  * If the implementation sets it to a value of lower than one, the
  59  * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
  60  * to return 0 otherwise.
  61  */
  62 static inline void
  63 __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
  64 {
  65         if (unlikely(atomic_xchg(count, 1) != 0))
  66                 fail_fn(count);
  67 }
  68
  69 #define __mutex_slowpath_needs_to_unlock()              0
  70
  71 /**
  72  * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  73  *
  74  *  @count: pointer of type atomic_t
  75  *  @fail_fn: spinlock based trylock implementation
  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         int prev = atomic_xchg(count, 0);
  90
  91         if (unlikely(prev < 0)) {
  92                 /*
  93                  * The lock was marked contended so we must restore that
  94                  * state. If while doing so we get back a prev value of 1
  95                  * then we just own it.
  96                  *
  97                  * [ In the rare case of the mutex going to 1, to 0, to -1
  98                  *   and then back to 0 in this few-instructions window,
  99                  *   this has the potential to trigger the slowpath for the
 100                  *   owner's unlock path needlessly, but that's not a problem
 101                  *   in practice. ]
 102                  */
 103                 prev = atomic_xchg(count, prev);
 104                 if (prev < 0)
 105                         prev = 0;
 106         }
 107
 108         return prev;
 109 }
 110
 111 #endif