Re: [PATCH] mtd: fix lock hierarchy in deregister_mtd_blktrans

There is a lock hierarchy of major_names_lock --> mtd_table_mutex. One
existing chain is as follows:

1. major_names_lock --> loop_ctl_mutex (when blk_request_module calls
loop_probe)

2. loop_ctl_mutex --> bdev->bd_mutex (when loop_control_ioctl calls
loop_remove, which then calls del_gendisk)

3. bdev->bd_mutex --> mtd_table_mutex (when blkdev_get_by_dev calls
__blkdev_get, which then calls blktrans_open)

Since unregister_blkdev grabs the major_names_lock, we need to call it
outside the critical section for mtd_table_mutex, otherwise we invert
the lock hierarchy.

Reported-by: Hillf Danton <redacted>
Signed-off-by: Desmond Cheong Zhi Xi <redacted>
---
  drivers/mtd/mtd_blkdevs.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index 7d26cfe24d05..87e26788ef60 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c

@@ -563,8 +563,8 @@ int deregister_mtd_blktrans(struct mtd_blktrans_ops *tr)
  	list_for_each_entry_safe(dev, next, &tr->devs, list)
  		tr->remove_dev(dev);
  
-	unregister_blkdev(tr->major, tr->name);
  	mutex_unlock(&mtd_table_mutex);
+	unregister_blkdev(tr->major, tr->name);
  
  	BUG_ON(!list_empty(&tr->devs));
  	return 0;