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