Re: [PATCH v2 3/3] io_uring/uring_cmd: avoid double indirect call in task work dispatch

Ming Lei
2025-10-24T03:42:30Z

On Thu, Oct 23, 2025 at 02:18:30PM -0600, Caleb Sander Mateos wrote:
> io_uring task work dispatch makes an indirect call to struct io_kiocb's
> io_task_work.func field to allow running arbitrary task work functions.
> In the uring_cmd case, this calls io_uring_cmd_work(), which immediately
> makes another indirect call to struct io_uring_cmd's task_work_cb field.
> Define the uring_cmd task work callbacks as functions whose signatures
> match io_req_tw_func_t. Define a IO_URING_CMD_TASK_WORK_ISSUE_FLAGS
> constant in io_uring/cmd.h to avoid manufacturing issue_flags in the
> uring_cmd task work callbacks. Now uring_cmd task work dispatch makes a
> single indirect call to the uring_cmd implementation's callback. This
> also allows removing the task_work_cb field from struct io_uring_cmd,
> freeing up some additional storage space.

The idea looks good.

>
> Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
> ---
> block/ioctl.c | 4 +++-
> drivers/block/ublk_drv.c | 15 +++++++++------
> drivers/nvme/host/ioctl.c | 5 +++--
> fs/btrfs/ioctl.c | 4 +++-
> fs/fuse/dev_uring.c | 5 +++--
> include/linux/io_uring/cmd.h | 16 +++++++---------
> io_uring/uring_cmd.c | 13 ++-----------
> 7 files changed, 30 insertions(+), 32 deletions(-)
>
> diff --git a/block/ioctl.c b/block/ioctl.c
> index d7489a56b33c..5c10d48fab27 100644
> --- a/block/ioctl.c
> +++ b/block/ioctl.c
> @@ -767,13 +767,15 @@ long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg)
> struct blk_iou_cmd {
> int res;
> bool nowait;
> };
>
> -static void blk_cmd_complete(struct io_uring_cmd *cmd, unsigned int issue_flags)
> +static void blk_cmd_complete(struct io_kiocb *req, io_tw_token_t tw)
> {
> + struct io_uring_cmd *cmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
> struct blk_iou_cmd *bic = io_uring_cmd_to_pdu(cmd, struct blk_iou_cmd);
> + unsigned int issue_flags = IO_URING_CMD_TASK_WORK_ISSUE_FLAGS;

Now `io_kiocb` is exposed to driver, it could be perfect if 'io_uring_cmd'
is kept in kernel API interface, IMO.

...

> diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> index b84b97c21b43..3efad93404f9 100644
> --- a/include/linux/io_uring/cmd.h
> +++ b/include/linux/io_uring/cmd.h
> @@ -9,18 +9,13 @@
> /* only top 8 bits of sqe->uring_cmd_flags for kernel internal use */
> #define IORING_URING_CMD_CANCELABLE (1U << 30)
> /* io_uring_cmd is being issued again */
> #define IORING_URING_CMD_REISSUE (1U << 31)
>
> -typedef void (*io_uring_cmd_tw_t)(struct io_uring_cmd *cmd,
> - unsigned issue_flags);
> -
> struct io_uring_cmd {
> struct file *file;
> const struct io_uring_sqe *sqe;
> - /* callback to defer completions to task context */
> - io_uring_cmd_tw_t task_work_cb;
> u32 cmd_op;
> u32 flags;
> u8 pdu[32]; /* available inline for free use */

pdu[40]

Thanks,
Ming