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

Roman Gushchin
2021-06-08T01:31:17Z

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-AlSwsSmVLrQ@public.gmane.org>
Signed-off-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>
---
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