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

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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.

Jens Axboe

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