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.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Roman Gushchin <redacted>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 7d2891d7ac12..b6fc13a4962d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c

@@ -1000,6 +1000,12 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr,
  */
 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

-- 
2.31.1