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

[Jens Axboe <axboe@kernel.dk>]

On 9/18/23 10:02 AM, Ming Lei wrote:
> 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.

Perfect, thanks Ming!

-- 
Jens Axboe