Re: [PATCH 00/10] io_uring/ublk: exit notifier support

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

On Mon, Sep 18, 2023 at 08:15:07AM -0600, Jens Axboe wrote:
> On 9/18/23 7:24 AM, Ming Lei wrote:
> > On Mon, Sep 18, 2023 at 06:54:33AM -0600, Jens Axboe wrote:
> >> On 9/17/23 10:10 PM, Ming Lei wrote:
> >>> Hello,
> >>>
> >>> In do_exit(), io_uring needs to wait pending requests.
> >>>
> >>> ublk is one uring_cmd driver, and its usage is a bit special by submitting
> >>> command for waiting incoming block IO request in advance, so if there
> >>> isn't any IO request coming, the command can't be completed. So far ublk
> >>> driver has to bind its queue with one ublk daemon server, meantime
> >>> starts one monitor work to check if this daemon is live periodically.
> >>> This way requires ublk queue to be bound one single daemon pthread, and
> >>> not flexible, meantime the monitor work is run in 3rd context, and the
> >>> implementation is a bit tricky.
> >>>
> >>> The 1st 3 patches adds io_uring task exit notifier, and the other
> >>> patches converts ublk into this exit notifier, and the implementation
> >>> becomes more robust & readable, meantime it becomes easier to relax
> >>> the ublk queue/daemon limit in future, such as not require to bind
> >>> ublk queue with single daemon.
> >>
> >> The normal approach for this is to ensure that each request is
> >> cancelable, which we need for other things too (like actual cancel
> >> support) Why can't we just do the same for ublk?
> > 
> > I guess you meant IORING_OP_ASYNC_CANCEL, which needs userspace to
> > submit this command, but here the userspace(ublk server) may be just panic
> > or killed, and there isn't chance to send IORING_OP_ASYNC_CANCEL.
> 
> Either that, or cancel done because of task exit.
> 
> > And driver doesn't have any knowledge if the io_uring ctx or io task
> > is exiting, so can't complete issued commands, then hang in
> > io_uring_cancel_generic() when the io task/ctx is exiting.
> 
> If you hooked into the normal cancel paths, you very much would get
> notified when the task is exiting. That's how the other cancelations
> work, eg if a task has pending poll requests and exits, they get
> canceled and reaped.

Ok, got the idea, thanks for the point!

Turns out it is cancelable uring_cmd, and I will try to work towards
this direction, and has got something in mind about the implementation.


Thanks, 
Ming