Re: [PATCH v8 2/8] writeback, cgroup: add smp_mb() to cgroup_writeback_umount()

Jan Kara
2021-06-08T08:43:48Z

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 <guro@fb.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> 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 <jack@suse.com>
SUSE Labs, CR