On Fri, 2013-02-01 at 12:23 +0200, Pekka Enberg wrote:
> On Wed, Jan 23, 2013 at 11:45 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> > The variables accessed in slab_alloc are volatile and therefore
> > the page pointer passed to node_match can be NULL. The processing
> > of data in slab_alloc is tentative until either the cmpxhchg
> > succeeds or the __slab_alloc slowpath is invoked. Both are
> > able to perform the same allocation from the freelist.
> >
> > Check for the NULL pointer in node_match.
> >
> > A false positive will lead to a retry of the loop in __slab_alloc.
> >
> > Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
>
> Steven, how did you trigger the problem - i.e. is this -rt only
> problem? Does the patch work for you?
I haven't tested Christoph's version yet. I've only tested my own. But
I'll take his and run them through tests as well. This bug is not easy
to hit.
It is not a -rt only bug, and yes it probably should go to stable. The
race is extremely small, but -rt creates scenarios that may only be hit
by 1000 CPU core machines. Because of the preemptive nature of -rt, -rt
is much more susceptible to race conditions than mainline. But these are
real bugs for mainline too. It may only trigger once a year, where in
-rt it will trigger once a week.
-- Steve
>
> > Index: linux/mm/slub.c
> > ===================================================================
> > --- linux.orig/mm/slub.c 2013-01-18 08:47:29.198954250 -0600
> > +++ linux/mm/slub.c 2013-01-18 08:47:40.579126371 -0600
> > @@ -2041,7 +2041,7 @@ static void flush_all(struct kmem_cache
> > static inline int node_match(struct page *page, int node)
> > {
> > #ifdef CONFIG_NUMA
> > - if (node != NUMA_NO_NODE && page_to_nid(page) != node)
> > + if (!page || (node != NUMA_NO_NODE && page_to_nid(page) != node))
> > return 0;
> > #endif
> > return 1;
> >
> > --
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