get a deadlock when insmod ftl

hi Brian Norris & David Woodhouse:
we found this deadlock with kernel version 4.1. then we found it maybe also exist after saw the code on 4.10. when insmod the ftl module, which register_mtd_blktrans.

-> register_mtd_blktrans()
-> mutex_lock(&mtd_table_mutex)
-> ftl_add_mtd()
-> add_mtd_blktrans_dev()
-> device_add_disk()
-> register_disk()
-> blkdev_get()
-> __blkdev_get()
-> blktrans_open()
-> mutex_lock(&mtd_table_mutex); !!!!!!!

the hung task log in attachment.

do you have any good ideas?
thanks,thanks!