linux-kernel - Re: [PATCH v4 4/4] ublk: mark ublk_queue as const for ublk_handle_need_get

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Message-ID: <CADUfDZosV8M3YOeJ90WK3R8o5OdMN1QKJyi1J-70hh3Vj+JiTA@mail.gmail.com>
Date: Wed, 16 Apr 2025 12:26:02 -0700
From: Caleb Sander Mateos <csander@...estorage.com>
To: Uday Shankar <ushankar@...estorage.com>
Cc: Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] ublk: mark ublk_queue as const for ublk_handle_need_get_data

On Tue, Apr 15, 2025 at 6:00 PM Uday Shankar <ushankar@...estorage.com> 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@...hat.com>
> Signed-off-by: Uday Shankar <ushankar@...estorage.com>
> ---
>  drivers/block/ublk_drv.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index e2cb54895481aebaa91ab23ba05cf26a950a642f..c8ce9349ca280b8b16040a1242a62b895ee01b5d 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 +1924,21 @@ static int ublk_commit_and_fetch(const struct ublk_queue *ubq,
>         return -EIOCBQUEUED;
>  }
>
> +static int ublk_handle_need_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)

nit: I see this is matching the name of the opcode (I am not sure why
it has "need" in its name) and there is already a function named
"ublk_need_get_data". But maybe naming this function "ublk_get_data"
would be clearer?

> +{
> +       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 -EIOCBQUEUED;

Here too, I think a return value of 0 would be clearer.

Best,
Caleb