[PATCH -mm] slub: update cpu after new_slab()

[Hugh Dickins]

SLUB gave me a NULL pointer dereference in slab_alloc(), in the
slab_lock(page) of its "Current cpuslab is acceptable" block: cpu
1 had been looking at cpu_slab[2], which then went NULL beneath it.
Since new_slab() may reenable interrupts and sleep (when __GFP_WAIT),
we may get rescheduled on a different cpu: so need to reevaluate it.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>

--- 2.6.21-rc7-mm1/mm/slub.c	2007-04-24 20:26:48.000000000 +0100
+++ linux/mm/slub.c	2007-04-25 15:49:12.000000000 +0100
@@ -1234,11 +1234,12 @@ have_slab:
 
 	page = new_slab(s, gfpflags, node);
 	if (page) {
+		cpu = smp_processor_id();
 		if (s->cpu_slab[cpu]) {
 			/*
-			 * Someone else populated the cpu_slab while
-			 * we enabled interrupts. The page may not
-			 * be on the requested node.
+			 * Someone else populated the cpu_slab while we enabled
+			 * interrupts, or we have got scheduled on another cpu.
+			 * The page may not be on the requested node.
 			 */
 			if (node == -1 ||
 				page_to_nid(s->cpu_slab[cpu]) == node) {
-
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