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writeback, cgroup: add smp_mb() to cgroup_writeback_umount()

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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: Roman Gushchin <guro@fb.com>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Roman Gushchin 2021-06-28 19:35:44 -07:00 committed by Linus Torvalds
parent 4ade5867b4
commit 592fa00218
1 changed files with 6 additions and 0 deletions
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6
fs/fs-writeback.c
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@ -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