[PATCH] x86-64: fix asm constraints in i386 atomic_add_return

Since v->counter is both read and written, it should be an output as well
as an input for the asm.  The current code only gets away with this because
counter is volatile.  Also, according to Documents/atomic_ops.txt,
atomic_add_return should provide a memory barrier, in particular a compiler
barrier, so the asm should be marked as clobbering memory.

Test case:

#include <stdio.h>

typedef struct { int counter; } atomic_t; /* NB: no "volatile" */

#define ATOMIC_INIT(i)	{ (i) }

#define atomic_read(v)		((v)->counter)

static __inline__ int atomic_add_return(int i, atomic_t *v)
{
	int __i = i;

	__asm__ __volatile__(
		"lock; xaddl %0, %1;"
		:"=r"(i)
		:"m"(v->counter), "0"(i));
/*	__asm__ __volatile__(
		"lock; xaddl %0, %1"
		:"+r" (i), "+m" (v->counter)
		: : "memory"); */
	return i + __i;
}

int main (void) {
	atomic_t a = ATOMIC_INIT(0);
	int x;

	x = atomic_add_return (1, &a);
	if ((x!=1) || (atomic_read(&a)!=1))
		printf("fail: %i, %i\n", x, atomic_read(&a));
}

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>

diff --git a/include/asm-i386/atomic.h b/include/asm-i386/atomic.h
index 51a166242522ab9ab9fe5d998986a3ff8637e24e..6aab7a105fad1e34bb7f92a65170ba3c1d02b91d 100644 (file)
--- a/include/asm-i386/atomic.h
+++ b/include/asm-i386/atomic.h
@@ -187,9 +187,9 @@ static __inline__ int atomic_add_return(int i, atomic_t *v)
        /* Modern 486+ processor */
        __i = i;
        __asm__ __volatile__(
-               LOCK_PREFIX "xaddl %0, %1;"
-               :"=r"(i)
-               :"m"(v->counter), "0"(i));
+               LOCK_PREFIX "xaddl %0, %1"
+               :"+r" (i), "+m" (v->counter)
+               : : "memory");
        return i + __i;
 
 #ifdef CONFIG_M386