linux/include/asm-generic/mutex-xchg.h
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   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(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 */
  48static 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 */
  69static 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 */
  93static 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
 119
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