raid5: check faulty flag for array status during recovery.

When we have more than 1 drive failure, it's possible we start
rebuild one drive while leaving another faulty drive in array.
To determine whether array will be optimal after building, current
code only check whether a drive is missing, which could potentially
lead to data corruption. This patch is to add checking Faulty flag.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5121,12 +5121,17 @@ static inline sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int
 		schedule_timeout_uninterruptible(1);
 	}
 	/* Need to check if array will still be degraded after recovery/resync
-	 * We don't need to check the 'failed' flag as when that gets set,
-	 * recovery aborts.
+	 * Note in case of > 1 drive failures it's possible we're rebuilding
+	 * one drive while leaving another faulty drive in array.
 	 */
-	for (i = 0; i < conf->raid_disks; i++)
-		if (conf->disks[i].rdev == NULL)
+	rcu_read_lock();
+	for (i = 0; i < conf->raid_disks; i++) {
+		struct md_rdev *rdev = ACCESS_ONCE(conf->disks[i].rdev);
+
+		if (rdev == NULL || test_bit(Faulty, &rdev->flags))
 			still_degraded = 1;
+	}
+	rcu_read_unlock();
 
 	bitmap_start_sync(mddev->bitmap, sector_nr, &sync_blocks, still_degraded);