md raid: fix hang when stopping arrays with metadata through dm-raid

When using device-mapper's dm-raid target, stopping a RAID array can cause
the system to hang under specific conditions.

This occurs when:

- A dm-raid managed device tree is suspended from top to bottom
   (the top-level RAID device is suspended first, followed by its
    underlying metadata and data devices)

- The top-level RAID device is then removed

Removing the top-level device triggers a hang in the following sequence:
the dm-raid destructor calls md_stop(), which tries to flush the
write-intent bitmap by writing to the metadata sub-devices. However, these
devices are already suspended, making them unable to complete the write-intent
operations and causing an indefinite block.

Fix:

- Prevent bitmap flushing when md_stop() is called from dm-raid
destructor context
  and avoid a quiescing/unquescing cycle which could also cause I/O

- Still allow write-intent bitmap flushing when called from dm-raid
suspend context

This ensures that RAID array teardown can complete successfully even when the
underlying devices are in a suspended state.

This second patch uses md_is_rdwr() to distinguish between suspend and
destructor paths as elaborated on above.

Link: https://lore.kernel.org/linux-raid/CAM23VxqYrwkhKEBeQrZeZwQudbiNey2_8B_SEOLqug=pXxaFrA@mail.gmail.com
Signed-off-by: Heinz Mauelshagen <[email protected]>
Signed-off-by: Yu Kuai <[email protected]>

Author: mauelsha

drivers/md/md.c

@@ -6851,13 +6851,15 @@ static void __md_stop_writes(struct mddev *mddev)
 {
 	timer_delete_sync(&mddev->safemode_timer);
 
-	if (mddev->pers && mddev->pers->quiesce) {
-		mddev->pers->quiesce(mddev, 1);
-		mddev->pers->quiesce(mddev, 0);
-	}
+	if (md_is_rdwr(mddev) || !mddev_is_dm(mddev)) {
+		if (mddev->pers && mddev->pers->quiesce) {
+			mddev->pers->quiesce(mddev, 1);
+			mddev->pers->quiesce(mddev, 0);
+		}
 
-	if (md_bitmap_enabled(mddev, true))
-		mddev->bitmap_ops->flush(mddev);
+		if (md_bitmap_enabled(mddev, true))
+			mddev->bitmap_ops->flush(mddev);
+	}
 
 	if (md_is_rdwr(mddev) &&
 	    ((!mddev->in_sync && !mddev_is_clustered(mddev)) ||