Re: [PATCH v3] block: genhd: don't call probe function with major_names_lock held

[Christoph Hellwig <hch@lst.de>]

This is the wrong way to approach it.  Instead of making things ever
more complex try to make it simpler.  In this case, ensure that the
destroy_workqueue is not held with pointless locks.  Given that the
loop device already is known to not have a reference and marked as in
the rundown state there shouldn't be anything that is required to
be protected by lo_mutex.  So something like this untested patch
should probably do the work:

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index fa1c298a8cfb..c734dc768316 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1347,16 +1347,15 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	 * became visible.
 	 */
 
-	mutex_lock(&lo->lo_mutex);
 	if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
 		err = -ENXIO;
-		goto out_unlock;
+		goto out;
 	}
 
 	filp = lo->lo_backing_file;
 	if (filp == NULL) {
 		err = -EINVAL;
-		goto out_unlock;
+		goto out;
 	}
 
 	if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags))
@@ -1366,6 +1365,8 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	blk_mq_freeze_queue(lo->lo_queue);
 
 	destroy_workqueue(lo->workqueue);
+
+	mutex_lock(&lo->lo_mutex);
 	spin_lock_irq(&lo->lo_work_lock);
 	list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
 				idle_list) {
@@ -1413,8 +1414,8 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
 	lo_number = lo->lo_number;
 	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
-out_unlock:
 	mutex_unlock(&lo->lo_mutex);
+
 	if (partscan) {
 		/*
 		 * open_mutex has been held already in release path, so don't
@@ -1435,7 +1436,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 		/* Device is gone, no point in returning error */
 		err = 0;
 	}
-
+out:
 	/*
 	 * lo->lo_state is set to Lo_unbound here after above partscan has
 	 * finished.