Comment 1 for bug 1551327

The problem is likely caused by:

[trusty] 5d2f571 bcache: allows use of register in udev to avoid "device_busy" error.
... which introduces a code path that calls mutex_unlock(&bch_register_lock) twice.

Another mainline commit (added in Linux v3.15, but not marked cc: stable) removes one of the unlocks, and is likely the fix we need for trusty and 3.13-stable:

[mainline] 4fa0340 bcache: Fix a lockdep splat in an error path