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=151211117608676Reported-by: NIvan Kozik <ivan@ludios.org> Reported-by: NAlban Browaeys <alban.browaeys@gmail.com> Tested-by: NMike Galbraith <efault@gmx.de> Signed-off-by: NPaolo Valente <paolo.valente@linaro.org> Signed-off-by: NSerena Ziviani <ziviani.serena@gmail.com> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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