Re: [PATCH] block: mq-deadline: Fix queue restart handling

[Sasha Levin <sashal@kernel.org>]

On Sat, Oct 05, 2019 at 12:03:18PM +0900, Damien Le Moal wrote:
>[ Upstream commit cb8acabbe33b110157955a7425ee876fb81e6bbc ]
>
>Commit 7211aef86f79 ("block: mq-deadline: Fix write completion
>handling") added a call to blk_mq_sched_mark_restart_hctx() in
>dd_dispatch_request() to make sure that write request dispatching does
>not stall when all target zones are locked. This fix left a subtle race
>when a write completion happens during a dispatch execution on another
>CPU:
>
>CPU 0: Dispatch			CPU1: write completion
>
>dd_dispatch_request()
>    lock(&dd->lock);
>    ...
>    lock(&dd->zone_lock);	dd_finish_request()
>    rq = find request		lock(&dd->zone_lock);
>    unlock(&dd->zone_lock);
>    				zone write unlock
>				unlock(&dd->zone_lock);
>				...
>				__blk_mq_free_request
>                                      check restart flag (not set)
>				      -> queue not run
>    ...
>    if (!rq && have writes)
>        blk_mq_sched_mark_restart_hctx()
>    unlock(&dd->lock)
>
>Since the dispatch context finishes after the write request completion
>handling, marking the queue as needing a restart is not seen from
>__blk_mq_free_request() and blk_mq_sched_restart() not executed leading
>to the dispatch stall under 100% write workloads.
>
>Fix this by moving the call to blk_mq_sched_mark_restart_hctx() from
>dd_dispatch_request() into dd_finish_request() under the zone lock to
>ensure full mutual exclusion between write request dispatch selection
>and zone unlock on write request completion.
>
>Fixes: 7211aef86f79 ("block: mq-deadline: Fix write completion handling")
>Cc: stable@vger.kernel.org
>Reported-by: Hans Holmberg <Hans.Holmberg@wdc.com>
>Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
>Reviewed-by: Christoph Hellwig <hch@lst.de>
>Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
>Signed-off-by: Jens Axboe <axboe@kernel.dk>

I've queued it up for 4.19, thanks!

-- 
Thanks,
Sasha