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                 /*
  30                  * We failed to acquire the lock, so mark it contended
  31                  * to ensure that any waiting tasks are woken up by the
  32                  * unlock slow path.
  33                  */
  34                 if (likely(atomic_xchg_acquire(count, -1) != 1))
  35                         fail_fn(count);
  36 }
  37
  38 /**
  39  *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
  40  *                                 from 1 to a 0 value
  41  *  @count: pointer of type atomic_t
  42  *
  43  * Change the count from 1 to a value lower than 1. This function returns 0
  44  * if the fastpath succeeds, or -1 otherwise.
  45  */
  46 static inline int
  47 __mutex_fastpath_lock_retval(atomic_t *count)
  48 {
  49         if (unlikely(atomic_xchg_acquire(count, 0) != 1))
  50                 if (likely(atomic_xchg(count, -1) != 1))
  51                         return -1;
  52         return 0;
  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 static inline void
  68 __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
  69 {
  70         if (unlikely(atomic_xchg_release(count, 1) != 0))
  71                 fail_fn(count);
  72 }
  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;
  95
  96         if (atomic_read(count) != 1)
  97                 return 0;
  98
  99         prev = atomic_xchg_acquire(count, 0);
 100         if (unlikely(prev < 0)) {
 101                 /*
 102                  * The lock was marked contended so we must restore that
 103                  * state. If while doing so we get back a prev value of 1
 104                  * then we just own it.
 105                  *
 106                  * [ In the rare case of the mutex going to 1, to 0, to -1
 107                  *   and then back to 0 in this few-instructions window,
 108                  *   this has the potential to trigger the slowpath for the
 109                  *   owner's unlock path needlessly, but that's not a problem
 110                  *   in practice. ]
 111                  */
 112                 prev = atomic_xchg_acquire(count, prev);
 113                 if (prev < 0)
 114                         prev = 0;
 115         }
 116
 117         return prev;
 118 }
 119
 120 #endif