The patch titled
Subject: writeback, cgroup: add smp_mb() to cgroup_writeback_umount()
has been added to the -mm tree. Its filename is
writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount.patch
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------------------------------------------------------
From: Roman Gushchin <guro@xxxxxx>
Subject: writeback, cgroup: add smp_mb() to cgroup_writeback_umount()
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.
Link: https://lkml.kernel.org/r/20210608230225.2078447-3-guro@xxxxxx
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/fs-writeback.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/fs/fs-writeback.c~writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount
+++ a/fs/fs-writeback.c
@@ -1000,6 +1000,12 @@ out_bdi_put:
*/
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
_
Patches currently in -mm which might be from guro@xxxxxx are
writeback-cgroup-do-not-switch-inodes-with-i_will_free-flag.patch
writeback-cgroup-add-smp_mb-to-cgroup_writeback_umount.patch
writeback-cgroup-increment-isw_nr_in_flight-before-grabbing-an-inode.patch
writeback-cgroup-switch-to-rcu_work-api-in-inode_switch_wbs.patch
writeback-cgroup-keep-list-of-inodes-attached-to-bdi_writeback.patch
writeback-cgroup-split-out-the-functional-part-of-inode_switch_wbs_work_fn.patch
writeback-cgroup-support-switching-multiple-inodes-at-once.patch
writeback-cgroup-release-dying-cgwbs-by-switching-attached-inodes.patch
