Unregister sync_thread doesn't need to hold reconfig_mutex since it
doesn't reconfigure array.
And it could cause deadlock problem for raid5 as follows:
1. process A tried to reap sync thread with reconfig_mutex held after echo
idle to sync_action.
2. raid5 sync thread was blocked if there were too many active stripes.
3. SB_CHANGE_PENDING was set (because of write IO comes from upper layer)
which causes the number of active stripes can't be decreased.
4. SB_CHANGE_PENDING can't be cleared since md_check_recovery was not able
to hold reconfig_mutex.
More details in the link:
issu://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@xxxxxxxxxxxxx/T/#t
Reported-and-tested-by: Donald Buczek <buczek@xxxxxxxxxxxxx>
Signed-off-by: Guoqing Jiang <guoqing.jiang@xxxxxxxxxxxxxxx>
---
drivers/md/md.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/md/md.c b/drivers/md/md.c
index ca40942..eec8c27 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9365,13 +9365,18 @@ void md_check_recovery(struct mddev *mddev)
EXPORT_SYMBOL(md_check_recovery);
void md_reap_sync_thread(struct mddev *mddev)
+ __releases(&mddev->reconfig_mutex)
+ __acquires(&mddev->reconfig_mutex)
+
{
struct md_rdev *rdev;
sector_t old_dev_sectors = mddev->dev_sectors;
bool is_reshaped = false;
/* resync has finished, collect result */
+ mddev_unlock(mddev);
md_unregister_thread(&mddev->sync_thread);
+ mddev_lock_nointr(mddev);
if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
!test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
mddev->degraded != mddev->raid_disks) {
--
2.7.4
