LXR linux/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 */
  25static 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        else
  31                smp_mb();
  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 */
  44static 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 */
  67static inline void
  68__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
  69{
  70        smp_mb();
  71        if (unlikely(atomic_xchg(count, 1) != 0))
  72                fail_fn(count);
  73}
  74
  75#define __mutex_slowpath_needs_to_unlock()              0
  76
  77/**
  78 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  79 *
  80 *  @count: pointer of type atomic_t
  81 *  @fail_fn: spinlock based trylock implementation
  82 *
  83 * Change the count from 1 to a value lower than 1, and return 0 (failure)
  84 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
  85 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
  86 * Additionally, if the value was < 0 originally, this function must not leave
  87 * it to 0 on failure.
  88 *
  89 * If the architecture has no effective trylock variant, it should call the
  90 * <fail_fn> spinlock-based trylock variant unconditionally.
  91 */
  92static inline int
  93__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  94{
  95        int prev = atomic_xchg(count, 0);
  96
  97        if (unlikely(prev < 0)) {
  98                /*
  99                 * The lock was marked contended so we must restore that
 100                 * state. If while doing so we get back a prev value of 1
 101                 * then we just own it.
 102                 *
 103                 * [ In the rare case of the mutex going to 1, to 0, to -1
 104                 *   and then back to 0 in this few-instructions window,
 105                 *   this has the potential to trigger the slowpath for the
 106                 *   owner's unlock path needlessly, but that's not a problem
 107                 *   in practice. ]
 108                 */
 109                prev = atomic_xchg(count, prev);
 110                if (prev < 0)
 111                        prev = 0;
 112        }
 113        smp_mb();
 114
 115        return prev;
 116}
 117
 118#endif
 119