[PATCH] nvme: fix nvme_ns_remove() deadlock

[keith.busch@intel.com (Keith Busch)]

On Mon, Apr 18, 2016@11:21:43AM -0700, Ming Lin wrote:
> From: Ming Lin <ming.l at ssi.samsung.com>
> 
> On receipt of a namespace attribute changed AER, we acquire the
> namespace mutex lock before proceeding to scan and validate the
> namespace list. In case of namespace detach/delete command,
> nvme_ns_remove function deadlocks trying to acquire the already held
> lock.
> 
> All callers, except nvme_remove_namespaces(), of nvme_ns_remove()
> already held namespaces_mutex. So we can simply fix the deadlock by
> not acquiring the mutex in nvme_ns_remove() and acquiring it in
> nvme_remove_namespaces().

This fixes one deadlock by introducing another.

The reason we don't hold the namespace lock during nvme_ns_remove is
because del_gendisk blocks until all IO is flushed. If the controller
fails during this, you'll deadlock nvme_dev_disable as it tries to
recover.