slab: NUMA slab allocator migration bugfix

NUMA slab allocator cpu migration bugfix

The NUMA slab allocator (specifically, cache_alloc_refill)
is not refreshing its local copies of what cpu and what
numa node it is on, when it drops and reacquires the irq
block that it inherited from its caller.  As a result
those values become invalid if an attempt to migrate the
process to another numa node occured while the irq block
had been dropped.

The solution is to make cache_alloc_refill reload these
variables whenever it drops and reacquires the irq block.

The error is very difficult to hit.  When it does occur,
one gets the following oops + stack traceback bits in
check_spinlock_acquired:

	kernel BUG at mm/slab.c:2417
	cache_alloc_refill+0xe6
	kmem_cache_alloc+0xd0
	...

This patch was developed against 2.6.23, ported to and
compiled-tested only against 2.6.25-rc4.
Signed-off-by: Joe Korty <joe.korty@ccur.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>

mm/slab.c

@@ -2964,11 +2964,10 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
 	struct array_cache *ac;
 	int node;
 
-	node = numa_node_id();
-
+retry:
 	check_irq_off();
+	node = numa_node_id();
 	ac = cpu_cache_get(cachep);
-retry:
 	batchcount = ac->batchcount;
 	if (!ac->touched && batchcount > BATCHREFILL_LIMIT) {
 		/*