Re: [PATCH 13/20] ublk: optimize ublk_user_copy() on daemon task

Ming Lei
2025-12-26T02:51:20Z

On Tue, Dec 16, 2025 at 10:34:47PM -0700, Caleb Sander Mateos wrote:
> ublk user copy syscalls may be issued from any task, so they take a
> reference count on the struct ublk_io to check whether it is owned by
> the ublk server and prevent a concurrent UBLK_IO_COMMIT_AND_FETCH_REQ
> from completing the request. However, if the user copy syscall is issued
> on the io's daemon task, a concurrent UBLK_IO_COMMIT_AND_FETCH_REQ isn't
> possible, so the atomic reference count dance is unnecessary. Check for
> UBLK_IO_FLAG_OWNED_BY_SRV to ensure the request is dispatched to the
> sever and obtain the request from ublk_io's req field instead of looking
> it up on the tagset. Skip the reference count increment and decrement.
> Commit 8a8fe42d765b ("ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon
> task") made an analogous optimization for ublk zero copy buffer
> registration.
>
> Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming