[PATCH] slab: fix drain_array() so that it works correctly with the shared_array

The list_lock also protects the shared array and we call drain_array() with
the shared array.  Therefore we cannot go as far as I wanted to but have to
take the lock in a way so that it also protects the array_cache in
drain_pages.

(Note: maybe we should make the array_cache locking more consistent?  I.e.
always take the array cache lock for shared arrays and disable interrupts
for the per cpu arrays?)
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -3521,7 +3521,8 @@ static void enable_cpucache(struct kmem_cache *cachep)
 
 /*
  * Drain an array if it contains any elements taking the l3 lock only if
- * necessary.
+ * necessary. Note that the l3 listlock also protects the array_cache
+ * if drain_array() is used on the shared array.
  */
 void drain_array(struct kmem_cache *cachep, struct kmem_list3 *l3,
 			 struct array_cache *ac, int force, int node)
@@ -3532,17 +3533,19 @@ void drain_array(struct kmem_cache *cachep, struct kmem_list3 *l3,
 		return;
 	if (ac->touched && !force) {
 		ac->touched = 0;
-	} else if (ac->avail) {
+	} else {
+		spin_lock_irq(&l3->list_lock);
+		if (ac->avail) {
 			tofree = force ? ac->avail : (ac->limit + 4) / 5;
 			if (tofree > ac->avail)
 				tofree = (ac->avail + 1) / 2;
-		spin_lock_irq(&l3->list_lock);
 			free_block(cachep, ac->entry, tofree, node);
-		spin_unlock_irq(&l3->list_lock);
 			ac->avail -= tofree;
 			memmove(ac->entry, &(ac->entry[tofree]),
 				sizeof(void *) * ac->avail);
 		}
+		spin_unlock_irq(&l3->list_lock);
+	}
 }
 
 /**