barrier.h

来自「linux 内核源代码」· C头文件 代码 · 共 142 行

/*
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 *
 * Copyright (C) 2006 by Ralf Baechle ([e-mail])
 */
#ifndef __ASM_BARRIER_H
#define __ASM_BARRIER_H

/*
 * read_barrier_depends - Flush all pending reads that subsequents reads
 * depend on.
 *
 * No data-dependent reads from memory-like regions are ever reordered
 * over this barrier.  All reads preceding this primitive are guaranteed
 * to access memory (but not necessarily other CPUs' caches) before any
 * reads following this primitive that depend on the data return by
 * any of the preceding reads.  This primitive is much lighter weight than
 * rmb() on most CPUs, and is never heavier weight than is
 * rmb().
 *
 * These ordering constraints are respected by both the local CPU
 * and the compiler.
 *
 * Ordering is not guaranteed by anything other than these primitives,
 * not even by data dependencies.  See the documentation for
 * memory_barrier() for examples and URLs to more information.
 *
 * For example, the following code would force ordering (the initial
 * value of "a" is zero, "b" is one, and "p" is "&a"):
 *
 * <programlisting>
 *	CPU 0				CPU 1
 *
 *	b = 2;
 *	memory_barrier();
 *	p = &b;				q = p;
 *					read_barrier_depends();
 *					d = *q;
 * </programlisting>
 *
 * because the read of "*q" depends on the read of "p" and these
 * two reads are separated by a read_barrier_depends().  However,
 * the following code, with the same initial values for "a" and "b":
 *
 * <programlisting>
 *	CPU 0				CPU 1
 *
 *	a = 2;
 *	memory_barrier();
 *	b = 3;				y = b;
 *					read_barrier_depends();
 *					x = a;
 * </programlisting>
 *
 * does not enforce ordering, since there is no data dependency between
 * the read of "a" and the read of "b".  Therefore, on some CPUs, such
 * as Alpha, "y" could be set to 3 and "x" to 0.  Use rmb()
 * in cases like this where there are no data dependencies.
 */

#define read_barrier_depends()		do { } while(0)
#define smp_read_barrier_depends()	do { } while(0)

#ifdef CONFIG_CPU_HAS_SYNC
#define __sync()				\
	__asm__ __volatile__(			\
		".set	push\n\t"		\
		".set	noreorder\n\t"		\
		".set	mips2\n\t"		\
		"sync\n\t"			\
		".set	pop"			\
		: /* no output */		\
		: /* no input */		\
		: "memory")
#else
#define __sync()	do { } while(0)
#endif

#define __fast_iob()				\
	__asm__ __volatile__(			\
		".set	push\n\t"		\
		".set	noreorder\n\t"		\
		"lw	$0,%0\n\t"		\
		"nop\n\t"			\
		".set	pop"			\
		: /* no output */		\
		: "m" (*(int *)CKSEG1)		\
		: "memory")

#define fast_wmb()	__sync()
#define fast_rmb()	__sync()
#define fast_mb()	__sync()
#define fast_iob()				\
	do {					\
		__sync();			\
		__fast_iob();			\
	} while (0)

#ifdef CONFIG_CPU_HAS_WB

#include <asm/wbflush.h>

#define wmb()		fast_wmb()
#define rmb()		fast_rmb()
#define mb()		wbflush()
#define iob()		wbflush()

#else /* !CONFIG_CPU_HAS_WB */

#define wmb()		fast_wmb()
#define rmb()		fast_rmb()
#define mb()		fast_mb()
#define iob()		fast_iob()

#endif /* !CONFIG_CPU_HAS_WB */

#if defined(CONFIG_WEAK_ORDERING) && defined(CONFIG_SMP)
#define __WEAK_ORDERING_MB	"       sync	\n"
#else
#define __WEAK_ORDERING_MB	"		\n"
#endif
#if defined(CONFIG_WEAK_REORDERING_BEYOND_LLSC) && defined(CONFIG_SMP)
#define __WEAK_LLSC_MB		"       sync	\n"
#else
#define __WEAK_LLSC_MB		"		\n"
#endif

#define smp_mb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")
#define smp_rmb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")
#define smp_wmb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")

#define set_mb(var, value) \
	do { var = value; smp_mb(); } while (0)

#define smp_llsc_mb()	__asm__ __volatile__(__WEAK_LLSC_MB : : :"memory")
#define smp_llsc_rmb()	__asm__ __volatile__(__WEAK_LLSC_MB : : :"memory")
#define smp_llsc_wmb()	__asm__ __volatile__(__WEAK_LLSC_MB : : :"memory")

#endif /* __ASM_BARRIER_H */