Re: [PATCH V3 1/2] blk-mq: introduce blk_mq_complete_request_sync()

[Keith Busch <kbusch@kernel.org>]

On Mon, Apr 08, 2019 at 05:40:46PM +0800, Ming Lei wrote:
> In NVMe's error handler, follows the typical steps of tearing down
> hardware for recovering controller:
> 
> 1) stop blk_mq hw queues
> 2) stop the real hw queues
> 3) cancel in-flight requests via
> 	blk_mq_tagset_busy_iter(tags, cancel_request, ...)
> cancel_request():
> 	mark the request as abort
> 	blk_mq_complete_request(req);
> 4) destroy real hw queues
> 
> However, there may be race between #3 and #4, because blk_mq_complete_request()
> may run q->mq_ops->complete(rq) remotelly and asynchronously, and
> ->complete(rq) may be run after #4.
> 
> This patch introduces blk_mq_complete_request_sync() for fixing the
> above race.
> 
> Cc: Keith Busch <kbusch@kernel.org>
> Cc: Sagi Grimberg <sagi@grimberg.me>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: James Smart <james.smart@broadcom.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: linux-nvme@lists.infradead.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c         | 11 +++++++++++
>  include/linux/blk-mq.h |  1 +
>  2 files changed, 12 insertions(+)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a9354835cf51..d8d89f3514ac 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -654,6 +654,17 @@ bool blk_mq_complete_request(struct request *rq)
>  }
>  EXPORT_SYMBOL(blk_mq_complete_request);
>  
> +bool blk_mq_complete_request_sync(struct request *rq)
> +{
> +	if (unlikely(blk_should_fake_timeout(rq->q)))
> +		return false;
> +
> +	WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
> +	rq->q->mq_ops->complete(rq);
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_complete_request_sync);

Could we possibly drop the fake timeout in this path? We're using this
in error handling that is past pretending completing requests didn't
happen.

Otherwise this all looks good to me.