Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued

Ming Lei
2017-09-12T10:23:03Z

On Wed, Sep 13, 2017 at 01:14:35AM +0800, Jianchao Wang wrote:
> When free the driver tag of the next rq with I/O scheduler
> configured, it get the first entry of the list, however, at the
> moment, the failed rq has been requeued at the head of the list.
> The rq it gets is the failed rq not the next rq.
> Free the driver tag of next rq before the failed one is requeued
> in the failure branch of queue_rq callback and it is just needed
> there.

Looks a good catch.

>
> Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
> ---
> block/blk-mq.c | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4603b11..19f848e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -983,7 +983,7 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
> bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
> {
> struct blk_mq_hw_ctx *hctx;
> - struct request *rq;
> + struct request *rq, *nxt;
> int errors, queued;
>
> if (list_empty(list))
> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
> if (list_empty(list))
> bd.last = true;
> else {
> - struct request *nxt;
> -
> nxt = list_first_entry(list, struct request, queuelist);
> bd.last = !blk_mq_get_driver_tag(nxt, NULL, false);
> }
>
> ret = q->mq_ops->queue_rq(hctx, &bd);
> if (ret == BLK_STS_RESOURCE) {
> + /*
> + * If an I/O scheduler has been configured and we got a
> + * driver tag for the next request already, free it again.
> + */
> + if (!list_empty(list)) {
> + nxt = list_first_entry(list, struct request, queuelist);
> + blk_mq_put_driver_tag(nxt);
> + }

The following way might be more simple and clean:

if (nxt)
blk_mq_put_driver_tag(nxt);

meantime 'nxt' need to be cleared inside the 'if (list_empty(list))'
before .queue_rq().

--
Ming