Re: [PATCH v5 2/4] ublk: mark ublk_queue as const for ublk_commit_and_fetch

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

On Wed, Apr 16, 2025 at 01:46:06PM -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 when handling COMMIT_AND_FETCH.
> Move the logic for this command into its own function
> ublk_commit_and_fetch. Also open code ublk_commit_completion in
> ublk_commit_and_fetch to reduce the number of parameters/avoid a
> redundant lookup.
> 
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
> ---
>  drivers/block/ublk_drv.c | 91 +++++++++++++++++++++++-------------------------
>  1 file changed, 43 insertions(+), 48 deletions(-)
> 
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 4e4863d873e6593ad94c72cbf971f792df5795ae..b44cbcbcc9d1735c398dc9ac7e93f4c8736b9201 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -1518,30 +1518,6 @@ static int ublk_ch_mmap(struct file *filp, struct vm_area_struct *vma)
>  	return remap_pfn_range(vma, vma->vm_start, pfn, sz, vma->vm_page_prot);
>  }
>  
> -static void ublk_commit_completion(struct ublk_device *ub,
> -		const struct ublksrv_io_cmd *ub_cmd)
> -{
> -	u32 qid = ub_cmd->q_id, tag = ub_cmd->tag;
> -	struct ublk_queue *ubq = ublk_get_queue(ub, qid);
> -	struct ublk_io *io = &ubq->ios[tag];
> -	struct request *req;
> -
> -	/* now this cmd slot is owned by nbd driver */
> -	io->flags &= ~UBLK_IO_FLAG_OWNED_BY_SRV;
> -	io->res = ub_cmd->result;
> -
> -	/* find the io request and complete */
> -	req = blk_mq_tag_to_rq(ub->tag_set.tags[qid], tag);
> -	if (WARN_ON_ONCE(unlikely(!req)))
> -		return;
> -
> -	if (req_op(req) == REQ_OP_ZONE_APPEND)
> -		req->__sector = ub_cmd->zone_append_lba;
> -
> -	if (likely(!blk_should_fake_timeout(req->q)))
> -		ublk_put_req_ref(ubq, req);
> -}
> -
>  /*
>   * Called from io task context via cancel fn, meantime quiesce ublk
>   * blk-mq queue, so we are called exclusively with blk-mq and io task
> @@ -1918,6 +1894,45 @@ static int ublk_unregister_io_buf(struct io_uring_cmd *cmd,
>  	return io_buffer_unregister_bvec(cmd, index, issue_flags);
>  }
>  
> +static int ublk_commit_and_fetch(const struct ublk_queue *ubq,
> +				 struct ublk_io *io, struct io_uring_cmd *cmd,
> +				 const struct ublksrv_io_cmd *ub_cmd,
> +				 struct request *req)

request argument can be removed by getting it inside this function directly,
and it is helpful to switch to retrieve & store it from 'ublk_io' later.

Otherwise, looks nice cleanup.


thanks,
Ming