/*
* Assembly implementation of the mutex fastpath, based on atomic
* decrement/increment.
*
* started by Ingo Molnar:
*
* Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
*/
#ifndef _ASM_MUTEX_H
#define _ASM_MUTEX_H
#include "asm/alternative.h"
/**
* __mutex_fastpath_lock - try to take the lock by moving the count
* from 1 to a 0 value
* @count: pointer of type atomic_t
* @fn: function to call if the original value was not 1
*
* Change the count from 1 to a value lower than 1, and call <fn> if it
* wasn't 1 originally. This function MUST leave the value lower than 1
* even when the "1" assertion wasn't true.
*/
#define __mutex_fastpath_lock(count, fail_fn) \
do { \
unsigned int dummy; \
\
typecheck(atomic_t *, count); \
typecheck_fn(fastcall void (*)(atomic_t *), fail_fn); \
\
__asm__ __volatile__( \
LOCK_PREFIX " decl (%%eax) \n" \
" js 2f \n" \
"1: \n" \
\
LOCK_SECTION_START("") \
"2: call "#fail_fn" \n" \
" jmp 1b \n" \
LOCK_SECTION_END \
\
:"=a" (dummy) \
: "a" (count) \
: "memory", "ecx", "edx"); \
} while (0)
/**
* __mutex_fastpath_lock_retval - try to take the lock by moving the count
* from 1 to a 0 value
* @count: pointer of type atomic_t
* @fail_fn: function to call if the original value was not 1
*
* Change the count from 1 to a value lower than 1, and call <fail_fn> if it
* wasn't 1 originally. This function returns 0 if the fastpath succeeds,
* or anything the slow path function returns
*/
static inline int
__mutex_fastpath_lock_retval(atomic_t *count,
int fastcall (*fail_fn)(atomic_t *))
{
if (unlikely(atomic_dec_return(count) < 0))
return fail_fn(count);
else
return 0;
}
/**
* __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
* @count: pointer of type atomic_t
* @fail_fn: function to call if the original value was not 0
*
* try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
* In the failure case, this function is allowed to either set the value
* to 1, or to set it to a value lower than 1.
*
* If the implementation sets it to a value of lower than 1, the
* __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
* to return 0 otherwise.
*/
#define __mutex_fastpath_unlock(count, fail_fn) \
do { \
unsigned int dummy; \
\
typecheck(atomic_t *, count); \
typecheck_fn(fastcall void (*)(atomic_t *), fail_fn); \
\
__asm__ __volatile__( \
LOCK_PREFIX " incl (%%eax) \n" \
" jle 2f \n" \
"1: \n" \
\
LOCK_SECTION_START("") \
"2: call "#fail_fn" \n" \
" jmp 1b \n" \
LOCK_SECTION_END \
\
:"=a" (dummy) \
: "a" (count) \
: "memory", "ecx", "edx"); \
} while (0)
#define __mutex_slowpath_needs_to_unlock() 1
/**
* __mutex_fastpath_trylock - try to acquire the mutex, without waiting
*
* @count: pointer of type atomic_t
* @fail_fn: fallback function
*
* Change the count from 1 to a value lower than 1, and return 0 (failure)
* if it wasn't 1 originally, or return 1 (success) otherwise. This function
* MUST leave the value lower than 1 even when the "1" assertion wasn't true.
* Additionally, if the value was < 0 originally, this function must not leave
* it to 0 on failure.
*/
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
/*
* We have two variants here. The cmpxchg based one is the best one
* because it never induce a false contention state. It is included
* here because architectures using the inc/dec algorithms over the
* xchg ones are much more likely to support cmpxchg natively.
*
* If not we fall back to the spinlock based variant - that is
* just as efficient (and simpler) as a 'destructive' probing of
* the mutex state would be.
*/
#ifdef __HAVE_ARCH_CMPXCHG
if (likely(atomic_cmpxchg(count, 1, 0) == 1))
return 1;
return 0;
#else
return fail_fn(count);
#endif
}
#endif
