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