ARM体系架构下的同步操作

是一个可选的条件代码（请参阅条件执行）。

是存放返回状态的目标寄存器。

是要加载或存储的寄存器。

为进行双字加载或存储时要用到的第二个寄存器。

是内存地址所基于的寄存器。

为应用于 Rn 中的值的可选偏移量。offset 只可用于 Thumb-2 指令中。 如果省略 offset，则认为偏移量为 0。

    MOV r1, #0x1                ; load the ‘lock taken’ value
try
    LDREX r0, [LockAddr]        ; load the lock value
    CMP r0, #0                  ; is the lock free?
    STREXEQ r0, r1, [LockAddr]  ; try and claim the lock
    CMPEQ r0, #0                ; did this succeed?
    BNE try                     ; no – try again
    ....                        ; yes – we have the lock

/*
*  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 
#include 
#include 

#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)  ((v)->counter)
#define atomic_set(v,i) (((v)->counter) = (i))

#if __LINUX_ARM_ARCH__ >= 6

/*
* 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;

       __asm__ __volatile__("@ atomic_add\n"
"1:     ldrex   %0, [%2]\n"
"       add     %0, %0, %3\n"
"       strex   %1, %0, [%2]\n"
"       teq     %1, #0\n"
"       bne     1b"
       : "=&r" (result), "=&r" (tmp)
       : "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, [%2]\n"
"       add     %0, %0, %3\n"
"       strex   %1, %0, [%2]\n"
"       teq     %1, #0\n"
"       bne     1b"
       : "=&r" (result), "=&r" (tmp)
       : "r" (&v->counter), "Ir" (i)
       : "cc");

       smp_mb();

       return result;
}

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

       __asm__ __volatile__("@ atomic_sub\n"
"1:     ldrex   %0, [%2]\n"
"       sub     %0, %0, %3\n"
"       strex   %1, %0, [%2]\n"
"       teq     %1, #0\n"
"       bne     1b"
       : "=&r" (result), "=&r" (tmp)
       : "r" (&v->counter), "Ir" (i)
       : "cc");
}

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

       smp_mb();

       __asm__ __volatile__("@ atomic_sub_return\n"
"1:     ldrex   %0, [%2]\n"
"       sub     %0, %0, %3\n"
"       strex   %1, %0, [%2]\n"
"       teq     %1, #0\n"
"       bne     1b"
       : "=&r" (result), "=&r" (tmp)
       : "r" (&v->counter), "Ir" (i)
       : "cc");

       smp_mb();

       return result;
}

static inline int atomic_cmpxchg(atomic_t *ptr, int old, int new)
{
       unsigned long oldval, res;

       smp_mb();

       do {
               __asm__ __volatile__("@ atomic_cmpxchg\n"
               "ldrex  %1, [%2]\n"
               "mov    %0, #0\n"
               "teq    %1, %3\n"
               "strexeq %0, %4, [%2]\n"
                   : "=&r" (res), "=&r" (oldval)
                   : "r" (&ptr->counter), "Ir" (old), "r" (new)
                   : "cc");
       } while (res);

       smp_mb();

       return oldval;
}

static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
{
       unsigned long tmp, tmp2;

       __asm__ __volatile__("@ atomic_clear_mask\n"
"1:     ldrex   %0, [%2]\n"
"       bic     %0, %0, %3\n"
"       strex   %1, %0, [%2]\n"
"       teq     %1, #0\n"
"       bne     1b"
       : "=&r" (tmp), "=&r" (tmp2)
       : "r" (addr), "Ir" (mask)
       : "cc");
}

#else /* ARM_ARCH_6 */

#ifdef CONFIG_SMP
#error SMP not supported on pre-ARMv6 CPUs
#endif

static inline int atomic_add_return(int i, atomic_t *v)
{
       unsigned long flags;
       int val;

       raw_local_irq_save(flags);
       val = v->counter;
       v->counter = val += i;
       raw_local_irq_restore(flags);

       return val;
}
#define atomic_add(i, v)        (void) atomic_add_return(i, v)

static inline int atomic_sub_return(int i, atomic_t *v)
{
       unsigned long flags;
       int val;

       raw_local_irq_save(flags);
       val = v->counter;
       v->counter = val -= i;
       raw_local_irq_restore(flags);

       return val;
}
#define atomic_sub(i, v)        (void) atomic_sub_return(i, v)

static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
{
       int ret;
       unsigned long flags;

       raw_local_irq_save(flags);
       ret = v->counter;
       if (likely(ret == old))
               v->counter = new;
       raw_local_irq_restore(flags);

       return ret;
}

static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
{
       unsigned long flags;

       raw_local_irq_save(flags);
       *addr &= ~mask;
       raw_local_irq_restore(flags);
}

#endif /* __LINUX_ARM_ARCH__ */

#define atomic_xchg(v, new) (xchg(&((v)->counter), new))

static inline int atomic_add_unless(atomic_t *v, int a, int u)
{
       int c, old;

       c = atomic_read(v);
       while (c != u && (old = atomic_cmpxchg((v), c, c + a)) != c)
               c = old;
       return c != u;
}
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)

#define atomic_inc(v)           atomic_add(1, v)
#define atomic_dec(v)           atomic_sub(1, v)

#define atomic_inc_and_test(v)  (atomic_add_return(1, v) == 0)
#define atomic_dec_and_test(v)  (atomic_sub_return(1, v) == 0)
#define atomic_inc_return(v)    (atomic_add_return(1, v))
#define atomic_dec_return(v)    (atomic_sub_return(1, v))
#define atomic_sub_and_test(i, v) (atomic_sub_return(i, v) == 0)

#define atomic_add_negative(i,v) (atomic_add_return(i, v) < 0)

#define smp_mb__before_atomic_dec()     smp_mb()
#define smp_mb__after_atomic_dec()      smp_mb()
#define smp_mb__before_atomic_inc()     smp_mb()
#define smp_mb__after_atomic_inc()      smp_mb()

#include 
#endif
#endif