1/*
  2 * Assembly implementation of the mutex fastpath, based on atomic
  3 * decrement/increment.
  4 *
  5 * started by Ingo Molnar:
  6 *
  7 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  8 */
  9#ifndef _ASM_X86_MUTEX_32_H
 10#define _ASM_X86_MUTEX_32_H
 11
 12#include <asm/alternative.h>
 13
 14/**
 15 *  __mutex_fastpath_lock - try to take the lock by moving the count
 16 *                          from 1 to a 0 value
 17 *  @count: pointer of type atomic_t
 18 *  @fn: function to call if the original value was not 1
 19 *
 20 * Change the count from 1 to a value lower than 1, and call <fn> if it
 21 * wasn't 1 originally. This function MUST leave the value lower than 1
 22 * even when the "1" assertion wasn't true.
 23 */
 24#define __mutex_fastpath_lock(count, fail_fn)			\
 25do {								\
 26	unsigned int dummy;					\
 27								\
 28	typecheck(atomic_t *, count);				\
 29	typecheck_fn(void (*)(atomic_t *), fail_fn);		\
 30								\
 31	asm volatile(LOCK_PREFIX "   decl (%%eax)\n"		\
 32		     "   jns 1f	\n"				\
 33		     "   call " #fail_fn "\n"			\
 34		     "1:\n"					\
 35		     : "=a" (dummy)				\
 36		     : "a" (count)				\
 37		     : "memory", "ecx", "edx");			\
 38} while (0)
 39
 40
 41/**
 42 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
 43 *                                 from 1 to a 0 value
 44 *  @count: pointer of type atomic_t
 45 *  @fail_fn: function to call if the original value was not 1
 46 *
 47 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
 48 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
 49 * or anything the slow path function returns
 50 */
 51static inline int __mutex_fastpath_lock_retval(atomic_t *count,
 52					       int (*fail_fn)(atomic_t *))
 53{
 54	if (unlikely(atomic_dec_return(count) < 0))
 55		return fail_fn(count);
 56	else
 57		return 0;
 58}
 59
 60/**
 61 *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
 62 *  @count: pointer of type atomic_t
 63 *  @fail_fn: function to call if the original value was not 0
 64 *
 65 * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
 66 * In the failure case, this function is allowed to either set the value
 67 * to 1, or to set it to a value lower than 1.
 68 *
 69 * If the implementation sets it to a value of lower than 1, the
 70 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
 71 * to return 0 otherwise.
 72 */
 73#define __mutex_fastpath_unlock(count, fail_fn)			\
 74do {								\
 75	unsigned int dummy;					\
 76								\
 77	typecheck(atomic_t *, count);				\
 78	typecheck_fn(void (*)(atomic_t *), fail_fn);		\
 79								\
 80	asm volatile(LOCK_PREFIX "   incl (%%eax)\n"		\
 81		     "   jg	1f\n"				\
 82		     "   call " #fail_fn "\n"			\
 83		     "1:\n"					\
 84		     : "=a" (dummy)				\
 85		     : "a" (count)				\
 86		     : "memory", "ecx", "edx");			\
 87} while (0)
 88
 89#define __mutex_slowpath_needs_to_unlock()	1
 90
 91/**
 92 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
 93 *
 94 *  @count: pointer of type atomic_t
 95 *  @fail_fn: fallback function
 96 *
 97 * Change the count from 1 to a value lower than 1, and return 0 (failure)
 98 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
 99 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
100 * Additionally, if the value was < 0 originally, this function must not leave
101 * it to 0 on failure.
102 */
103static inline int __mutex_fastpath_trylock(atomic_t *count,
104					   int (*fail_fn)(atomic_t *))
105{
106	/*
107	 * We have two variants here. The cmpxchg based one is the best one
108	 * because it never induce a false contention state.  It is included
109	 * here because architectures using the inc/dec algorithms over the
110	 * xchg ones are much more likely to support cmpxchg natively.
111	 *
112	 * If not we fall back to the spinlock based variant - that is
113	 * just as efficient (and simpler) as a 'destructive' probing of
114	 * the mutex state would be.
115	 */
116#ifdef __HAVE_ARCH_CMPXCHG
117	if (likely(atomic_cmpxchg(count, 1, 0) == 1))
118		return 1;
119	return 0;
120#else
121	return fail_fn(count);
122#endif
123}
124
125#endif /* _ASM_X86_MUTEX_32_H */