md: fix md_write_start() deadlock w/o metadata devices

If no metadata devices are configured on raid1/4/5/6/10
(e.g. via dm-raid), md_write_start() unconditionally waits
for superblocks to be written thus deadlocking.

Fix introduces mddev->has_superblocks bool, defines it in md_run()
and checks for it in md_write_start() to conditionally avoid waiting.

Once on it, check for non-existing superblocks in md_super_write().

Link: https://bugzilla.kernel.org/show_bug.cgi?id=198647
Fixes: cc27b0c7 ("md: fix deadlock between mddev_suspend() and md_write_start()")
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>

drivers/md/md.c

@@ -801,6 +801,9 @@ void md_super_write(struct mddev *mddev, struct md_rdev *rdev,
 	struct bio *bio;
 	int ff = 0;
 
+	if (!page)
+		return;
+
 	if (test_bit(Faulty, &rdev->flags))
 		return;
 
@@ -5452,6 +5455,7 @@ int md_run(struct mddev *mddev)
 	 * the only valid external interface is through the md
 	 * device.
 	 */
+	mddev->has_superblocks = false;
 	rdev_for_each(rdev, mddev) {
 		if (test_bit(Faulty, &rdev->flags))
 			continue;
@@ -5465,6 +5469,9 @@ int md_run(struct mddev *mddev)
 				set_disk_ro(mddev->gendisk, 1);
 		}
 
+		if (rdev->sb_page)
+			mddev->has_superblocks = true;
+
 		/* perform some consistency tests on the device.
 		 * We don't want the data to overlap the metadata,
 		 * Internal Bitmap issues have been handled elsewhere.
@@ -8065,6 +8072,7 @@ EXPORT_SYMBOL(md_done_sync);
 bool md_write_start(struct mddev *mddev, struct bio *bi)
 {
 	int did_change = 0;
+
 	if (bio_data_dir(bi) != WRITE)
 		return true;
 
@@ -8097,6 +8105,8 @@ bool md_write_start(struct mddev *mddev, struct bio *bi)
 	rcu_read_unlock();
 	if (did_change)
 		sysfs_notify_dirent_safe(mddev->sysfs_state);
+	if (!mddev->has_superblocks)
+		return true;
 	wait_event(mddev->sb_wait,
 		   !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) ||
 		   mddev->suspended);

drivers/md/md.h

@@ -468,6 +468,8 @@ struct mddev {
 	void (*sync_super)(struct mddev *mddev, struct md_rdev *rdev);
 	struct md_cluster_info		*cluster_info;
 	unsigned int			good_device_nr;	/* good device num within cluster raid */
+
+	bool	has_superblocks:1;
 };
 
 enum recovery_flags {