Re: [PATCH 2/6] blk-mq: call blk_mq_start_request from ->queue_rq

[Ming Lei <tom.leiming@gmail.com>]

On Sun, Sep 14, 2014 at 7:40 AM, Christoph Hellwig <hch@lst.de> wrote:
> When we call blk_mq_start_request from the core blk-mq code before calling into
> ->queue_rq there is a racy window where the timeout handler can hit before we've
> fully set up the driver specific part of the command.

It is quite difficult to happen since the default timeout is 30s, which
is long enough.

Suppose it happens, what is the matter without setting up request's pdu
since the request can only be completed one time thanks to flag of
REQ_ATOM_COMPLETE?

>
> Move the call to blk_mq_start_request into the driver so the driver can start
> the request only once it is fully set up.

The change isn't friendly for drivers since every drivers need to do that and
it should have been done in blk-mq core code.

Also the timeout handler still may see pdu of request not setup because
writing rq in blk_mq_start_request() and writing on pdu may be reordered.

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/blk-mq.c                    | 13 ++++++-------
>  drivers/block/mtip32xx/mtip32xx.c |  2 ++
>  drivers/block/null_blk.c          |  2 ++
>  drivers/block/virtio_blk.c        |  2 ++
>  drivers/scsi/scsi_lib.c           |  1 +
>  include/linux/blk-mq.h            |  1 +
>  6 files changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 42c94c8..c3333ab 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -380,7 +380,7 @@ void blk_mq_complete_request(struct request *rq)
>  }
>  EXPORT_SYMBOL(blk_mq_complete_request);
>
> -static void blk_mq_start_request(struct request *rq)
> +void blk_mq_start_request(struct request *rq)
>  {
>         struct request_queue *q = rq->q;
>
> @@ -412,16 +412,18 @@ static void blk_mq_start_request(struct request *rq)
>                 rq->nr_phys_segments++;
>         }
>  }
> +EXPORT_SYMBOL(blk_mq_start_request);
>
>  static void __blk_mq_requeue_request(struct request *rq)
>  {
>         struct request_queue *q = rq->q;
>
>         trace_block_rq_requeue(q, rq);
> -       clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
>
> -       if (q->dma_drain_size && blk_rq_bytes(rq))
> -               rq->nr_phys_segments--;
> +       if (test_and_clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
> +               if (q->dma_drain_size && blk_rq_bytes(rq))
> +                       rq->nr_phys_segments--;
> +       }
>  }
>
>  void blk_mq_requeue_request(struct request *rq)
> @@ -729,8 +731,6 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>                 rq = list_first_entry(&rq_list, struct request, queuelist);
>                 list_del_init(&rq->queuelist);
>
> -               blk_mq_start_request(rq);
> -
>                 ret = q->mq_ops->queue_rq(hctx, rq, list_empty(&rq_list));
>                 switch (ret) {
>                 case BLK_MQ_RQ_QUEUE_OK:
> @@ -1177,7 +1177,6 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
>                 int ret;
>
>                 blk_mq_bio_to_request(rq, bio);
> -               blk_mq_start_request(rq);
>
>                 /*
>                  * For OK queue, we are done. For error, kill it. Any other
> diff --git a/drivers/block/mtip32xx/mtip32xx.c b/drivers/block/mtip32xx/mtip32xx.c
> index 0e2084f..4042440 100644
> --- a/drivers/block/mtip32xx/mtip32xx.c
> +++ b/drivers/block/mtip32xx/mtip32xx.c
> @@ -3783,6 +3783,8 @@ static int mtip_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *rq,
>         if (unlikely(mtip_check_unal_depth(hctx, rq)))
>                 return BLK_MQ_RQ_QUEUE_BUSY;
>
> +       blk_mq_start_request(rq);
> +
>         ret = mtip_submit_request(hctx, rq);
>         if (likely(!ret))
>                 return BLK_MQ_RQ_QUEUE_OK;
> diff --git a/drivers/block/null_blk.c b/drivers/block/null_blk.c
> index c5b7315..332ce20 100644
> --- a/drivers/block/null_blk.c
> +++ b/drivers/block/null_blk.c
> @@ -321,6 +321,8 @@ static int null_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *rq,
>         cmd->rq = rq;
>         cmd->nq = hctx->driver_data;
>
> +       blk_mq_start_request(rq);
> +
>         null_handle_cmd(cmd);
>         return BLK_MQ_RQ_QUEUE_OK;
>  }
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 13756e0..83816bf 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -205,6 +205,8 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req,
>                 }
>         }
>
> +       blk_mq_start_request(req);
> +
>         num = blk_rq_map_sg(hctx->queue, vbr->req, vbr->sg);
>         if (num) {
>                 if (rq_data_dir(vbr->req) == WRITE)
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 1ec00ba..b06a355 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1890,6 +1890,7 @@ static int scsi_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req,
>         scsi_init_cmd_errh(cmd);
>         cmd->scsi_done = scsi_mq_done;
>
> +       blk_mq_start_request(req);
>         reason = scsi_dispatch_cmd(cmd);
>         if (reason) {
>                 scsi_set_blocked(cmd, reason);
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 9c4e306..878b6f7 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -159,6 +159,7 @@ struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag);
>  struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *, const int ctx_index);
>  struct blk_mq_hw_ctx *blk_mq_alloc_single_hw_queue(struct blk_mq_tag_set *, unsigned int, int);
>
> +void blk_mq_start_request(struct request *rq);
>  void blk_mq_end_io(struct request *rq, int error);
>  void __blk_mq_end_io(struct request *rq, int error);
>
> --
> 1.9.1
>
> --
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-- 
Ming Lei