Messages in this thread Patch in this message | | | From | Roman Gushchin <> | Subject | [PATCH v9 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount() | Date | Tue, 8 Jun 2021 16:02:19 -0700 |
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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.
Suggested-by: Jan Kara <jack@suse.cz> Signed-off-by: Roman Gushchin <guro@fb.com> 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
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