On Mon 11-03-19 20:17:01, Laurent Dufour wrote:
> The commit 95402b382901 ("cpu-hotplug: replace per-subsystem mutexes with
> get_online_cpus()") remove the CPU_LOCK_ACQUIRE operation which was use to
> grap the cache_chain_mutex lock which was protecting cache_reap() against
> CPU hot plug operations.
>
> Later the commit 18004c5d4084 ("mm, sl[aou]b: Use a common mutex
> definition") changed cache_chain_mutex to slab_mutex but this didn't help
> fixing the missing the cache_reap() protection against CPU hot plug
> operations.
>
> Here we are stopping the per cpu worker while holding the slab_mutex to
> ensure that cache_reap() is not running in our back and will not be
> triggered anymore for this cpu.
>
> This patch fixes that race leading to SLAB's data corruption when CPU
> hotplug are triggered. We hit it while doing partition migration on PowerVM
> leading to CPU reconfiguration through the CPU hotplug mechanism.
What is the actual race? slab_offline_cpu calls cancel_delayed_work_sync
so it removes a pending item and waits for the item to finish if they run
concurently. So why do we need an additional lock?
> This fix is covering kernel containing to the commit 6731d4f12315 ("slab:
> Convert to hotplug state machine"), ie 4.9.1, earlier kernel needs a
> slightly different patch.
>
> Cc: sta...@vger.kernel.org
> Cc: Christoph Lameter <c...@linux.com>
> Cc: Pekka Enberg <penb...@kernel.org>
> Cc: David Rientjes <rient...@google.com>
> Cc: Joonsoo Kim <iamjoonsoo....@lge.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Signed-off-by: Laurent Dufour <lduf...@linux.ibm.com>
> ---
> mm/slab.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 28652e4218e0..ba499d90f27f 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1103,6 +1103,7 @@ static int slab_online_cpu(unsigned int cpu)
>
> static int slab_offline_cpu(unsigned int cpu)
> {
> + mutex_lock(&slab_mutex);
> /*
> * Shutdown cache reaper. Note that the slab_mutex is held so
> * that if cache_reap() is invoked it cannot do anything
> @@ -1112,6 +1113,7 @@ static int slab_offline_cpu(unsigned int cpu)
> cancel_delayed_work_sync(&per_cpu(slab_reap_work, cpu));
> /* Now the cache_reaper is guaranteed to be not running. */
> per_cpu(slab_reap_work, cpu).work.func = NULL;
> + mutex_unlock(&slab_mutex);
> return 0;
> }
>
> --
> 2.21.0
--
Michal Hocko
SUSE Labs
