md: enhance checking in md_check_recovery()

For md_check_recovery():

1) if 'MD_RECOVERY_RUNING' is not set, register new sync_thread.
2) if 'MD_RECOVERY_RUNING' is set:
 a) if 'MD_RECOVERY_DONE' is not set, don't do anything, wait for
   md_do_sync() to be done.
 b) if 'MD_RECOVERY_DONE' is set, unregister sync_thread. Current code
   expects that sync_thread is not NULL, otherwise new sync_thread will
   be registered, which will corrupt the array.

Make sure md_check_recovery() won't register new sync_thread if
'MD_RECOVERY_RUNING' is still set, and a new WARN_ON_ONCE() is added for
the above corruption,

Signed-off-by: Yu Kuai <[email protected]>
Reviewed-by: Xiao Ni <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: YuKuai-huawei

drivers/md/md.c

@@ -9388,16 +9388,24 @@ void md_check_recovery(struct mddev *mddev)
 		if (mddev->sb_flags)
 			md_update_sb(mddev, 0);
 
-		if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
-		    !test_bit(MD_RECOVERY_DONE, &mddev->recovery)) {
-			/* resync/recovery still happening */
-			clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
-			goto unlock;
-		}
-		if (mddev->sync_thread) {
+		/*
+		 * Never start a new sync thread if MD_RECOVERY_RUNNING is
+		 * still set.
+		 */
+		if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)) {
+			if (!test_bit(MD_RECOVERY_DONE, &mddev->recovery)) {
+				/* resync/recovery still happening */
+				clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
+				goto unlock;
+			}
+
+			if (WARN_ON_ONCE(!mddev->sync_thread))
+				goto unlock;
+
 			md_reap_sync_thread(mddev);
 			goto unlock;
 		}
+
 		/* Set RUNNING before clearing NEEDED to avoid
 		 * any transients in the value of "sync_action".
 		 */