Re: [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount()
From: Jan Kara <jack@suse.cz>
Date: 2021-06-08 08:43:51
Also in:
cgroups, linux-fsdevel, lkml
On Mon 07-06-21 18:31:17, Roman Gushchin wrote:
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>
Looks good. Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> Honza
quoted hunk ↗ jump to hunk
--- fs/fs-writeback.c | 6 ++++++ 1 file changed, 6 insertions(+)diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index bd99890599e0..3564efcc4b78 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
-- Jan Kara [off-list ref] SUSE Labs, CR