As Steven Rostedt has pointer out: Rescheduling could occur on a differnet processor
after the determination of the per cpu pointer and before the tid is retrieved.
This could result in allocation from the wrong node in slab_alloc.
The effect is much more severe in slab_free() where we could free to the freelist
of the wrong page.
The window for something like that occurring is pretty small but it is possible.
Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c 2013-01-23 15:06:39.805154107 -0600
+++ linux/mm/slub.c 2013-01-23 15:24:47.656868067 -0600
@@ -2331,13 +2331,18 @@ static __always_inline void *slab_alloc_
s = memcg_kmem_get_cache(s, gfpflags);
redo:
-
/*
* Must read kmem_cache cpu data via this cpu ptr. Preemption is
* enabled. We may switch back and forth between cpus while
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
+ *
+ * Preemption is disabled for the retrieval of the tid because that
+ * must occur from the current processor. We cannot allow rescheduling
+ * on a different processor between the determination of the pointer
+ * and the retrieval of the tid.
*/
+ preempt_disable();
c = __this_cpu_ptr(s->cpu_slab);
/*
@@ -2347,7 +2352,7 @@ redo:
* linked list in between.
*/
tid = c->tid;
- barrier();
+ preempt_enable();
object = c->freelist;
page = c->page;
@@ -2594,10 +2599,11 @@ redo:
* data is retrieved via this pointer. If we are on the same cpu
* during the cmpxchg then the free will succedd.
*/
+ preempt_disable();
c = __this_cpu_ptr(s->cpu_slab);
tid = c->tid;
- barrier();
+ preempt_enable();
if (likely(page == c->page)) {
set_freepointer(s, object, c->freelist);
--
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