include/asm-x86_64/mutex.h

/*
 * 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

/**
 * __mutex_fastpath_lock - decrement and call function if negative
 * @v: pointer of type atomic_t
 * @fail_fn: function to call if the result is negative
 *
 * Atomically decrements @v and calls <fail_fn> if the result is negative.
 */
#define __mutex_fastpath_lock(v, fail_fn)				\
do {									\
	unsigned long dummy;						\
									\
	typecheck(atomic_t *, v);					\
	typecheck_fn(fastcall void (*)(atomic_t *), fail_fn);		\
									\
	__asm__ __volatile__(						\
		LOCK_PREFIX "   decl (%%rdi)	\n"			\
			"   jns 1f		\n"			\
			"   call "#fail_fn"	\n"			\
			"1:"						\
									\
		:"=D" (dummy)						\
		: "D" (v)						\
		: "rax", "rsi", "rdx", "rcx",				\
		  "r8", "r9", "r10", "r11", "memory");			\
} 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 - increment and call function if nonpositive
 * @v: pointer of type atomic_t
 * @fail_fn: function to call if the result is nonpositive
 *
 * Atomically increments @v and calls <fail_fn> if the result is nonpositive.
 */
#define __mutex_fastpath_unlock(v, fail_fn)				\
do {									\
	unsigned long dummy;						\
									\
	typecheck(atomic_t *, v);					\
	typecheck_fn(fastcall void (*)(atomic_t *), fail_fn);		\
									\
	__asm__ __volatile__(						\
		LOCK_PREFIX "   incl (%%rdi)	\n"			\
			"   jg 1f		\n"			\
			"   call "#fail_fn"	\n"			\
			"1:			  "			\
									\
		:"=D" (dummy)						\
		: "D" (v)						\
		: "rax", "rsi", "rdx", "rcx",				\
		  "r8", "r9", "r10", "r11", "memory");			\
} 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 0 and return 1 (success), or return 0 (failure)
 * if it wasn't 1 originally. [the fallback function is never used on
 * x86_64, because all x86_64 CPUs have a CMPXCHG instruction.]
 */
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
	if (likely(atomic_cmpxchg(count, 1, 0) == 1))
		return 1;
	else
		return 0;
}

#endif