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

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

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.

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.


Thanks,
Ming