Re: Atomic operations

[H. Peter Anvin]

Followup to:  <EE83E551E08D1D43AD52D50B9F5110927E7A10@ntserver2>
By author:    Gregory Giguashvili <[e-mail]>
In newsgroup: linux.dev.kernel
>
> Hello,
> 
> I wonder if someone can help me to change the behaviour of the atomic
> functions available in <asm/atomic.h> include file. The operations I need to
> implement are described below:
> 
> atomic_t test_and_set (int i, atomic_t* v)
> {
>    atomic_t old = *v;
>    v->counter = i;
>    return old;
> }
> 

This is not a test and set operation.

On i386:

atomic_t atomic_exchange (atomic_t i, atomic_t *v)
{
   asm volatile("xchgl %0,%1" : "+m" (*v), "+r" (i));
   return i;
}

> atomic_t test_then_add (int i, atomic_t* v)
> {
>    atomic_t old = *v;
>    v->counter += i;
>    return old;
> }

There is no way to do this (without waiting and trying again type
code) that I know of on i386.  However, you can test for zeroness of
the result, or for <= 0, or a few other options.

int test_and_add (atomic_t i, atomic_t *v)
{
  char was_nonzero;	/* MUST BE CHAR!!! */

  asm volatile("lock; addl %2,%0; setz %1"
    : "+m" (*v), "=rm" (was_nonzero)
    : "g" (i));

  return was_nonzero;
}

  

-- 
<[e-mail]> at work, <[e-mail]> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<[e-mail]>