[PATCH] mm/slab: protect cache_reap() against CPU and memory hot plug operations

[Laurent Dufour <ldufour@linux.ibm.com>]

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.

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: stable@vger.kernel.org
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Laurent Dufour <ldufour@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