Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Andrew Morton]

On Mon, 7 Oct 2019 10:16:21 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> On Fri 04-10-19 14:57:01, Michal Hocko wrote:
> > On Fri 04-10-19 08:31:49, Qian Cai wrote:
> > > Long time ago, there fixed a similar deadlock in show_slab_objects()
> > > [1]. However, it is apparently due to the commits like 01fb58bcba63
> > > ("slab: remove synchronous synchronize_sched() from memcg cache
> > > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> > > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> > > just reading files in /sys/kernel/slab which will generate a lockdep
> > > splat below.
> > > 
> > > Since the "mem_hotplug_lock" here is only to obtain a stable online node
> > > mask while racing with NUMA node hotplug, in the worst case, the results
> > > may me miscalculated while doing NUMA node hotplug, but they shall be
> > > corrected by later reads of the same files.
> > 
> > I think it is important to mention that this doesn't expose the
> > show_slab_objects to use-after-free. There is only a single path that
> > might really race here and that is the slab hotplug notifier callback
> > __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
> > doesn't really destroy kmem_cache_node data structures.

Yes, I noted this during review.  It's a bit subtle and is worthy of
more than a changelog note, I think.  How about this?

--- a/mm/slub.c~mm-slub-fix-a-deadlock-in-show_slab_objects-fix
+++ a/mm/slub.c
@@ -4851,6 +4851,10 @@ static ssize_t show_slab_objects(struct
 	 * already held which will conflict with an existing lock order:
 	 *
 	 * mem_hotplug_lock->slab_mutex->kernfs_mutex
+	 *
+	 * We don't really need mem_hotplug_lock (to hold off
+	 * slab_mem_going_offline_callback()) here because slab's memory hot
+	 * unplug code doesn't destroy the kmem_cache->node[] data.
 	 */
 
 #ifdef CONFIG_SLUB_DEBUG
_

> Andrew, please add this to the changelog so that we do not have to
> scratch heads again when looking into that code.

I did that as well.