get a deadlock when insmod ftl

[Gu Zheng]

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!
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