From: Roman Gushchin <guro@xxxxxx> Subject: writeback, cgroup: add smp_mb() to cgroup_writeback_umount() A full memory barrier is required between clearing SB_ACTIVE flag in generic_shutdown_super() and checking isw_nr_in_flight in cgroup_writeback_umount(), otherwise a new switch operation might be scheduled after atomic_read(&isw_nr_in_flight) returned 0. This would result in a non-flushed isw_wq, and a potential crash. The problem hasn't yet been seen in the real life and was discovered by Jan Kara by looking into the code. Link: https://lkml.kernel.org/r/20210608230225.2078447-3-guro@xxxxxx Signed-off-by: Roman Gushchin <guro@xxxxxx> Suggested-by: Jan Kara <jack@xxxxxxx> Reviewed-by: Jan Kara <jack@xxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Dave Chinner <dchinner@xxxxxxxxxx> Cc: Dennis Zhou <dennis@xxxxxxxxxx> Cc: Jan Kara <jack@xxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/fs-writeback.c | 6 ++++++ 1 file changed, 6 insertions(+) --- a/fs/fs-writeback.c~writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount +++ a/fs/fs-writeback.c @@ -1000,6 +1000,12 @@ out_bdi_put: */ void cgroup_writeback_umount(void) { + /* + * SB_ACTIVE should be reliably cleared before checking + * isw_nr_in_flight, see generic_shutdown_super(). + */ + smp_mb(); + if (atomic_read(&isw_nr_in_flight)) { /* * Use rcu_barrier() to wait for all pending callbacks to _
