Re: ublk: Graceful Upgrade of ublk server application

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

On Tue, Apr 15, 2025 at 08:15:08AM +0000, Yoav Cohen wrote:
> I am seeking advice on whether it is possible to upgrade the ublksrv version without terminating the daemon abruptly. Specifically, I would like the daemon to exit gracefully, ensuring all necessary cleanups are performed.
> In my current implementation, I attempted to cancel all ublk uring SQEs (specifically the COMMIT_AND_FETCH_REQ/FETCH_REQ operations) using the following approach:
> io_uring_prep_cancel_fd(sqe, cdev_fd, IORING_ASYNC_CANCEL_ALL);io_uring_prep_cancel_fd(sqe, cdev_fd, IORING_ASYNC_CANCEL_ALL);

That shouldn't work because COMMIT_AND_FETCH_REQ/FETCH_REQ has been
issued to ublk driver already.

> However, this method does not seem to be effective. In my scenario, I have a single io_uring instance that serves multiple devices and other producers, so simply stopping the polling of CQEs is not a viable solution due to potential race conditions.
> Could you please provide any suggestions or guidance on how to achieve a graceful upgrade of the ublksrv version without causing disruptions?
>  

You can delete all ublk devices handled by this single io_ring instance
first by sending UBLK_CMD_DEL_DEV, then exit ublk server loop if there
isn't any pending uring_cmd & target IO. And the ublk server need to stop
to issue COMMIT_AND_FETCH_REQ after getting uring_cmd with UBLK_IO_RES_ABORT.

I guess you want to send the control command in single pthread too?

If yes, it still can work with coroutine or modern language's .async/await.

This feature is actually in my todo list for libublk-rs, just not started
because of not seeing real requirement.


Thanks,
Ming