block, bfq: add requeue-request hook

Commit 'a6a252e6 ("blk-mq-sched: decide how to handle flush rq via
RQF_FLUSH_SEQ")' makes all non-flush re-prepared requests for a device
be re-inserted into the active I/O scheduler for that device. As a
consequence, I/O schedulers may get the same request inserted again,
even several times, without a finish_request invoked on that request
before each re-insertion.

This fact is the cause of the failure reported in [1]. For an I/O
scheduler, every re-insertion of the same re-prepared request is
equivalent to the insertion of a new request. For schedulers like
mq-deadline or kyber, this fact causes no harm. In contrast, it
confuses a stateful scheduler like BFQ, which keeps state for an I/O
request, until the finish_request hook is invoked on the request. In
particular, BFQ may get stuck, waiting forever for the number of
request dispatches, of the same request, to be balanced by an equal
number of request completions (while there will be one completion for
that request). In this state, BFQ may refuse to serve I/O requests
from other bfq_queues. The hang reported in [1] then follows.

However, the above re-prepared requests undergo a requeue, thus the
requeue_request hook of the active elevator is invoked for these
requests, if set. This commit then addresses the above issue by
properly implementing the hook requeue_request in BFQ.

[1] https://marc.info/?l=linux-block&m=151211117608676



Reported-by: Ivan Kozik <ivan@ludios.org>
Reported-by: Alban Browaeys <alban.browaeys@gmail.com>
Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Serena Ziviani <ziviani.serena@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>