The CPU hotplug offline handler in the block layer checks for any
in-flight requests on a CPU going offline. It prevents the CPU hotplug
state engine from progressing as long as there are pending requests.

This is done by checking for any allocated requests on the hardware
context that is going offline. The driver is responsible for completing
all in-flight requests.

However, the driver might be performing error recovery simultaneously.
Therefore, the request queue might be in a frozen or quiesced state. In
this case, requests may not make progress (see
blk_mq_sched_dispatch_requests for an example).

Introduce an explicit handshake protocol between the driver and the
block layer. This allows the driver to signal when it is safe to ignore
any remaining pending requests.

Signed-off-by: Daniel Wagner <wagi@kernel.org>

…pus_evenly"

This reverts commit 0263f92.

The reason the lock was removed was that the nvme-pci driver reset
handler attempted to acquire the CPU read lock during CPU hotplug
offlining (holds the CPU write lock). Consequently, the block layer
offline notifier callback could not progress because in-flight requests
were detected.

Since then, in-flight detection has been improved, and the nvme-pci
driver now explicitly updates the hctx state when it is safe to ignore
detected in-flight requests. As a result, it's possible to reintroduce
the CPU read lock in group_cpus_evenly.

Signed-off-by: Daniel Wagner <wagi@kernel.org>