| Message ID | 20210608013123.1088882-3-guro@fb.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | cgroup, blkcg: prevent dirty inodes to pin dying memory cgroups | expand |
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 >
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
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> --- fs/fs-writeback.c | 6 ++++++ 1 file changed, 6 insertions(+)