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                /*
  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 */
  46static 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 */
  67static 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 */
  91static inline int
  92__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  93{
  94        int prev = atomic_xchg_acquire(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_acquire(count, prev);
 109                if (prev < 0)
 110                        prev = 0;
 111        }
 112
 113        return prev;
 114}
 115
 116#endif
 117