[PATCH] md: don't unregister sync_thread with reconfig_mutex held

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@molgen.mpg.de/T/#t

Reported-and-tested-by: Donald Buczek <redacted>
Signed-off-by: Guoqing Jiang <redacted>
---
 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