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}
 31
 32/**
 33 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
 34 *                                 from 1 to a 0 value
 35 *  @count: pointer of type atomic_t
 36 *  @fail_fn: function to call if the original value was not 1
 37 *
 38 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
 39 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
 40 * or anything the slow path function returns
 41 */
 42static inline int
 43__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
 44{
 45	if (unlikely(atomic_xchg(count, 0) != 1))
 46		return fail_fn(count);
 47	return 0;
 48}
 49
 50/**
 51 *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
 52 *  @count: pointer of type atomic_t
 53 *  @fail_fn: function to call if the original value was not 0
 54 *
 55 * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
 56 * In the failure case, this function is allowed to either set the value to
 57 * 1, or to set it to a value lower than one.
 58 * If the implementation sets it to a value of lower than one, the
 59 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
 60 * to return 0 otherwise.
 61 */
 62static inline void
 63__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
 64{
 65	if (unlikely(atomic_xchg(count, 1) != 0))
 66		fail_fn(count);
 67}
 68
 69#define __mutex_slowpath_needs_to_unlock()		0
 70
 71/**
 72 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
 73 *
 74 *  @count: pointer of type atomic_t
 75 *  @fail_fn: spinlock based trylock implementation
 76 *
 77 * Change the count from 1 to a value lower than 1, and return 0 (failure)
 78 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
 79 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
 80 * Additionally, if the value was < 0 originally, this function must not leave
 81 * it to 0 on failure.
 82 *
 83 * If the architecture has no effective trylock variant, it should call the
 84 * <fail_fn> spinlock-based trylock variant unconditionally.
 85 */
 86static inline int
 87__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
 88{
 89	int prev = atomic_xchg(count, 0);
 90
 91	if (unlikely(prev < 0)) {
 92		/*
 93		 * The lock was marked contended so we must restore that
 94		 * state. If while doing so we get back a prev value of 1
 95		 * then we just own it.
 96		 *
 97		 * [ In the rare case of the mutex going to 1, to 0, to -1
 98		 *   and then back to 0 in this few-instructions window,
 99		 *   this has the potential to trigger the slowpath for the
100		 *   owner's unlock path needlessly, but that's not a problem
101		 *   in practice. ]
102		 */
103		prev = atomic_xchg(count, prev);
104		if (prev < 0)
105			prev = 0;
106	}
107
108	return prev;
109}
110
111#endif