Re: [PATCH v5 4/4] ublk: mark ublk_queue as const for ublk_handle_need_get_data

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 16, 2025 at 01:46:08PM -0600, Uday Shankar wrote:
> We now allow multiple tasks to operate on I/Os belonging to the same
> queue concurrently. This means that any writes to ublk_queue in the I/O
> path are potential sources of data races. Try to prevent these by
> marking ublk_queue pointers as const in ublk_handle_need_get_data. Also
> move a bit more of the NEED_GET_DATA-specific logic into
> ublk_handle_need_get_data, to make the pattern in __ublk_ch_uring_cmd
> more uniform.
> 
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> ---
>  drivers/block/ublk_drv.c | 32 +++++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 215ab45b00e10150e58d7f5ea5b5d13e40a1aa79..5f9679c03305576bee586388cab82a6ea5472f8b 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -1291,7 +1291,7 @@ static void ublk_cmd_tw_cb(struct io_uring_cmd *cmd,
>  	ublk_dispatch_req(ubq, pdu->req, issue_flags);
>  }
>  
> -static void ublk_queue_cmd(struct ublk_queue *ubq, struct request *rq)
> +static void ublk_queue_cmd(const struct ublk_queue *ubq, struct request *rq)
>  {
>  	struct io_uring_cmd *cmd = ubq->ios[rq->tag].cmd;
>  	struct ublk_uring_cmd_pdu *pdu = ublk_get_uring_cmd_pdu(cmd);
> @@ -1813,15 +1813,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
>  	mutex_unlock(&ub->mutex);
>  }
>  
> -static void ublk_handle_need_get_data(struct ublk_device *ub, int q_id,
> -		int tag)
> -{
> -	struct ublk_queue *ubq = ublk_get_queue(ub, q_id);
> -	struct request *req = blk_mq_tag_to_rq(ub->tag_set.tags[q_id], tag);
> -
> -	ublk_queue_cmd(ubq, req);
> -}
> -
>  static inline int ublk_check_cmd_op(u32 cmd_op)
>  {
>  	u32 ioc_type = _IOC_TYPE(cmd_op);
> @@ -1933,6 +1924,20 @@ static int ublk_commit_and_fetch(const struct ublk_queue *ubq,
>  	return 0;
>  }
>  
> +static int ublk_get_data(const struct ublk_queue *ubq, struct ublk_io *io,
> +			 struct io_uring_cmd *cmd,
> +			 const struct ublksrv_io_cmd *ub_cmd,
> +			 struct request *req)
> +{
> +	if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> +		return -EINVAL;
> +
> +	ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
> +	ublk_queue_cmd(ubq, req);
> +
> +	return 0;
> +}
> +
>  static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
>  			       unsigned int issue_flags,
>  			       const struct ublksrv_io_cmd *ub_cmd)
> @@ -2026,10 +2031,11 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
>  			goto out;
>  		break;
>  	case UBLK_IO_NEED_GET_DATA:
> -		if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> +		ret = ublk_get_data(
> +			ubq, io, cmd, ub_cmd,
> +			blk_mq_tag_to_rq(ub->tag_set.tags[ub_cmd->q_id], tag));

Same with patch 2, moving request retrieval into ublk_get_data() could be
cleaner and helpful for switching to store request pointer into `ublk_io`.

Otherwise, looks nice cleanup.

Thanks,
Ming