On 8/27/19 10:40 PM, Damien Le Moal wrote:
> 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.
Applied, thanks.
--
Jens Axboe
