slub: tid must be retrieved from the percpu area of the current processor
As Steven Rostedt has pointer out: rescheduling could occur on a
different 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@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
diff --git a/mm/slub.c b/mm/slub.c
index 8b1b99d..4df2c0c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2332,13 +2332,18 @@
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);
/*
@@ -2348,7 +2353,7 @@
* linked list in between.
*/
tid = c->tid;
- barrier();
+ preempt_enable();
object = c->freelist;
page = c->page;
@@ -2595,10 +2600,11 @@
* 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);