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   1 /*
   2  * asm-generic/mutex-xchg.h
   3  *
   4  * Generic implementation of the mutex fastpath, based on xchg().
   5  *
   6  * NOTE: An xchg based implementation is 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 #define __mutex_fastpath_lock(count, fail_fn)                           \
  26 do {                                                                    \
  27         if (unlikely(atomic_xchg(count, 0) != 1))                       \
  28                 fail_fn(count);                                         \
  29         else                                                            \
  30                 smp_mb();                                               \
  31 } while (0)
  32
  33
  34 /**
  35  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  36  *                                 from 1 to a 0 value
  37  *  @count: pointer of type atomic_t
  38  *  @fail_fn: function to call if the original value was not 1
  39  *
  40  * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  41  * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  42  * or anything the slow path function returns
  43  */
  44 static inline int
  45 __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
  46 {
  47         if (unlikely(atomic_xchg(count, 0) != 1))
  48                 return fail_fn(count);
  49         else {
  50                 smp_mb();
  51                 return 0;
  52         }
  53 }
  54
  55 /**
  56  *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
  57  *  @count: pointer of type atomic_t
  58  *  @fail_fn: function to call if the original value was not 0
  59  *
  60  * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
  61  * In the failure case, this function is allowed to either set the value to
  62  * 1, or to set it to a value lower than one.
  63  * If the implementation sets it to a value of lower than one, the
  64  * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
  65  * to return 0 otherwise.
  66  */
  67 #define __mutex_fastpath_unlock(count, fail_fn)                         \
  68 do {                                                                    \
  69         smp_mb();                                                       \
  70         if (unlikely(atomic_xchg(count, 1) != 0))                       \
  71                 fail_fn(count);                                         \
  72 } while (0)
  73
  74 #define __mutex_slowpath_needs_to_unlock()              0
  75
  76 /**
  77  * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  78  *
  79  *  @count: pointer of type atomic_t
  80  *  @fail_fn: spinlock based trylock implementation
  81  *
  82  * Change the count from 1 to a value lower than 1, and return 0 (failure)
  83  * if it wasn't 1 originally, or return 1 (success) otherwise. This function
  84  * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
  85  * Additionally, if the value was < 0 originally, this function must not leave
  86  * it to 0 on failure.
  87  *
  88  * If the architecture has no effective trylock variant, it should call the
  89  * <fail_fn> spinlock-based trylock variant unconditionally.
  90  */
  91 static inline int
  92 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  93 {
  94         int prev = atomic_xchg(count, 0);
  95
  96         if (unlikely(prev < 0)) {
  97                 /*
  98                  * The lock was marked contended so we must restore that
  99                  * state. If while doing so we get back a prev value of 1
 100                  * then we just own it.
 101                  *
 102                  * [ In the rare case of the mutex going to 1, to 0, to -1
 103                  *   and then back to 0 in this few-instructions window,
 104                  *   this has the potential to trigger the slowpath for the
 105                  *   owner's unlock path needlessly, but that's not a problem
 106                  *   in practice. ]
 107                  */
 108                 prev = atomic_xchg(count, prev);
 109                 if (prev < 0)
 110                         prev = 0;
 111         }
 112         smp_mb();
 113
 114         return prev;
 115 }
 116
 117 #endif