[PATCH V3 1/2] md: don't unregister sync_thread with reconfig_mutex held

[Guoqing Jiang <guoqing.jiang@linux.dev>]

From: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>

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:
https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t

Let's call md_unregister_thread between mddev_unlock and mddev_lock if
reconfig_mutex is held, and mddev_is_locked is introduced accordingly.

Reported-by: Donald Buczek <buczek@molgen.mpg.de>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
---
v2: https://lore.kernel.org/all/1613177399-22024-1-git-send-email-guoqing.jiang@cloud.ionos.com/#t
v1: https://lore.kernel.org/all/1612923676-18294-1-git-send-email-guoqing.jiang@cloud.ionos.com/

 drivers/md/md.c | 9 ++++++++-
 drivers/md/md.h | 5 +++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 309b3af906ad..a70e7f0f9268 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9432,10 +9432,17 @@ void md_reap_sync_thread(struct mddev *mddev)
 {
 	struct md_rdev *rdev;
 	sector_t old_dev_sectors = mddev->dev_sectors;
-	bool is_reshaped = false;
+	bool is_reshaped = false, is_locked = false;
 
+	if (mddev_is_locked(mddev)) {
+		is_locked = true;
+		mddev_unlock(mddev);
+	}
 	/* resync has finished, collect result */
 	md_unregister_thread(&mddev->sync_thread);
+	if (is_locked)
+		mddev_lock(mddev);
+
 	if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
 	    !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
 	    mddev->degraded != mddev->raid_disks) {
diff --git a/drivers/md/md.h b/drivers/md/md.h
index 6ac283864533..af6f3978b62b 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -549,6 +549,11 @@ static inline int mddev_trylock(struct mddev *mddev)
 }
 extern void mddev_unlock(struct mddev *mddev);
 
+static inline int mddev_is_locked(struct mddev *mddev)
+{
+	return mutex_is_locked(&mddev->reconfig_mutex);
+}
+
 static inline void md_sync_acct(struct block_device *bdev, unsigned long nr_sectors)
 {
 	atomic_add(nr_sectors, &bdev->bd_disk->sync_io);
-- 
2.31.1