arm架构下的原子操作（atomic）函数源码分析

最新推荐文章于 2024-11-30 10:06:53 发布

duanlove

最新推荐文章于 2024-11-30 10:06:53 发布

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本文链接：https://blog.youkuaiyun.com/duanlove/article/details/7990839

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本文详细分析了在ARM架构下，如何使用ldrex和strexeq指令实现原子操作，如atomic_add、atomic_sub等。通过这些指令，确保在多核和单核环境下对内存的独占访问和更新，避免数据竞争问题。同时，文章还讨论了不同ARM版本下的实现差异，并提供了相关的源码示例。

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关于下面源码中用到的ldrex和strexeq指令请参考：

arm上ldrex和strexeq指令用来尝试获取独占内存权限和设置在独占权限时回写（http://blog.youkuaiyun.com/duanlove/article/details/8048050 ）

/*
* arch/arm/include/asm/atomic.h
*
* Copyright (C) 1996 Russell King.
* Copyright (C) 2002 Deep Blue Solutions Ltd.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#ifndef __ASM_ARM_ATOMIC_H
#define __ASM_ARM_ATOMIC_H

#include <linux/compiler.h>
#include <linux/types.h>
#include <asm/system.h>

#define ATOMIC_INIT(i) { (i) }

#ifdef __KERNEL__

/*
* On ARM, ordinary assignment (str instruction) doesn't clear the local
* strex/ldrex monitor on some implementations. The reason we can use it for
* atomic_set() is the clrex or dummy strex done on every exception return.
*/
#define atomic_read(v) (*(volatile int *)&(v)->counter)
#define atomic_set(v,i) (((v)->counter) = (i))

#if __LINUX_ARM_ARCH__ >= 6 // 如果ARM架构的版本大于等于v6

/*
* ARMv6 UP and SMP safe atomic ops. We use load exclusive and
* store exclusive to ensure that these are atomic. We may loop
* to ensure that the update happens.
*/
static inline void atomic_add(int i, atomic_t *v)
{
unsigned long tmp;
int result;

// strex 用于独占访问。

__asm__ __volatile__("@ atomic_add\n"
"1: ldrex %0, [%3]\n"
" add %0, %0, %4\n"
" strex %1, %0, [%3]\n"
" teq %1, #0\n"
" bne 1b"
: "=&r" (result), "=&r" (tmp), "+Qo" (v->counter)
: "r" (&v->counter), "Ir" (i)
: "cc");
}

static inline int atomic_add_return(int i, atomic_t *v)
{
unsigned long tmp;
int result;

smp_mb();

__asm__ __volatile__("@ atomic_add_return\n"
"1: ldrex %0, [%3]\n"
" add %0, %0, %4\n"
" strex %1, %0, [%3]\n"
" teq %1, #0\n"
" bne 1b"
: "=&r" (result), "=&r" (tmp), "+Qo" (v->counter)
: "r" (&v->counter), "Ir" (i)
: "cc");

smp_mb();

return result;
}

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