linux-kernel - Re: [PATCH v4 2/4] ublk: mark ublk_queue as const for ublk_commit_and

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Message-ID: <CADUfDZpkDUE8heSFEhA1aCfn3a59D1+=2piWPtRvX3t9FLgjbw@mail.gmail.com>
Date: Wed, 16 Apr 2025 11:59:25 -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 2/4] ublk: mark ublk_queue as const for ublk_commit_and_fetch

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 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@...hat.com>
> Signed-off-by: Uday Shankar <ushankar@...estorage.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 9a0d2547512fc8119460739230599d48d2c2a306..153f67d92248ad45bddd2437b1306bb23df7d1ae 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)
> +{
> +       if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> +               return -EINVAL;
> +
> +       if (ublk_need_map_io(ubq)) {
> +               /*
> +                * COMMIT_AND_FETCH_REQ has to provide IO buffer if
> +                * NEED GET DATA is not enabled or it is Read IO.
> +                */
> +               if (!ub_cmd->addr && (!ublk_need_get_data(ubq) ||
> +                                       req_op(req) == REQ_OP_READ))
> +                       return -EINVAL;
> +       } else if (req_op(req) != REQ_OP_ZONE_APPEND && ub_cmd->addr) {
> +               /*
> +                * User copy requires addr to be unset when command is
> +                * not zone append
> +                */
> +               return -EINVAL;
> +       }
> +
> +       ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
> +
> +       /* now this cmd slot is owned by ublk driver */
> +       io->flags &= ~UBLK_IO_FLAG_OWNED_BY_SRV;
> +       io->res = ub_cmd->result;
> +
> +       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);
> +
> +       return -EIOCBQUEUED;

I think it would be clearer to just return 0. __ublk_ch_uring_cmd()
already takes care of returning -EIOCBQUEUED in the successful case.
Aside from that,

Reviewed-by: Caleb Sander Mateos <csander@...estorage.com>