Re: Atomic operations

[Brian Gerst <bgerst@didntduck.org>]

Gregory Giguashvili wrote:
> 
> 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;
> }

What you have coded is really an exchange, not a test.  Here is the asm
equivalent of what you coded:

int atomic_xchg(int i, atomic_t *v)
{
	int ret;
	__asm__("xchgl %1,%0"
		: "=m" (v->counter), "=r" (ret)
		: "0" (v->counter), "1" (i));
	return ret;
}

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

int atomic_xadd(int i, atomic_t *v)
{
	int ret;
	__asm__(LOCK "xaddl %1,%0"
		: "=m" (v->counter), "=r" (ret)
		: "0" (v->counter), "1" (i));
	return ret;
}

This one only works on 486+, but there are practically no real 386 SMP
systems.

--

				Brian Gerst