On Mon, May 18, 2020 at 6:05 PM Waiman Long [off-list ref] wrote:

On 5/16/20 10:19 PM, Qian Cai wrote:

quoted

quoted

On Apr 27, 2020, at 7:56 PM, Waiman Long [off-list ref] wrote:

It turns out that switching from slab_mutex to memcg_cache_ids_sem in
slab_attr_store() does not completely eliminate circular locking dependency
as shown by the following lockdep splat when the system is shut down:

[ 2095.079697] Chain exists of:
[ 2095.079697]   kn->count#278 --> memcg_cache_ids_sem --> slab_mutex
[ 2095.079697]
[ 2095.090278]  Possible unsafe locking scenario:
[ 2095.090278]
[ 2095.096227]        CPU0                    CPU1
[ 2095.100779]        ----                    ----
[ 2095.105331]   lock(slab_mutex);
[ 2095.108486]                                lock(memcg_cache_ids_sem);
[ 2095.114961]                                lock(slab_mutex);
[ 2095.120649]   lock(kn->count#278);
[ 2095.124068]
[ 2095.124068]  *** DEADLOCK ***

Can you show the full splat?

quoted

To eliminate this possibility, we have to use trylock to acquire
memcg_cache_ids_sem. Unlikely slab_mutex which can be acquired in
many places, the memcg_cache_ids_sem write lock is only acquired
in memcg_alloc_cache_id() to double the size of memcg_nr_cache_ids.
So the chance of successive calls to memcg_alloc_cache_id() within
a short time is pretty low. As a result, we can retry the read lock
acquisition a few times if the first attempt fails.

Signed-off-by: Waiman Long <redacted>

The code looks a bit hacky and probably not that robust. Since it is the shutdown path which is not all that important without lockdep, maybe you could drop this single patch for now until there is a better solution?

That is true. Unlike using the slab_mutex, the chance of failing to
acquire a read lock on memcg_cache_ids_sem is pretty low. Maybe just
print_once a warning if that happen.

That seems cleaner. If you are going to repost this series, you could
also mention that the series will fix slabinfo triggering a splat as
well.