After patch commit f52f5c71f3d4b ("md: fix stopping sync thread"), dmraid
stops sync thread asynchronously. The calling process is:
dev_remove->dm_destroy->__dm_destroy->raid_postsuspend->raid_dtr

raid_postsuspend does two jobs. First, it stops sync thread. Then it
suspend array. Now it can stop sync thread successfully. But it doesn't
set MD_RECOVERY_FROZEN. It's introduced by patch f52f5c71f3d4b. So after
raid_postsuspend, the sync thread starts again. raid_dtr can't stop the
sync thread because the array is already suspended.

This can be reproduced easily by those commands:
while [ 1 ]; do
vgcreate test_vg /dev/loop0 /dev/loop1
lvcreate --type raid1 -L 400M -m 1 -n test_lv test_vg
lvchange -an test_vg
vgremove test_vg -ff
done

Fixes: f52f5c71f3d4 ("md: fix stopping sync thread")
Signed-off-by: Xiao Ni <redacted>
---
 drivers/md/md.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 54790261254d..77e3af7cf6bb 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -6337,6 +6337,7 @@ static void __md_stop_writes(struct mddev *mddev)
 void md_stop_writes(struct mddev *mddev)
 {
 	mddev_lock_nointr(mddev);
+	set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
 	__md_stop_writes(mddev);
 	mddev_unlock(mddev);
 }

-- 
2.32.0 (Apple Git-132)