Commit 592fa002 authored by Roman Gushchin's avatar Roman Gushchin Committed by Linus Torvalds
Browse files

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@fb.com


Signed-off-by: default avatarRoman Gushchin <guro@fb.com>
Suggested-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Jan Kara <jack@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4ade5867

fs/fs-writeback.c

+6 −0
Original line number Diff line number Diff line
@@ -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