Re: shutdown not affecting connection?

[Avi Kivity <avi@scylladb.com>]

On 2/8/20 8:42 PM, Glauber Costa wrote:
> Hi
>
> BTW, my apologies but I should have specified the kernel I am running:
> 90206ac99c1f25b7f7a4c2c40a0b9d4561ffa9bf
>
> On Sat, Feb 8, 2020 at 9:26 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>> Hi
>>
>> On 2/8/2020 4:55 PM, Glauber Costa wrote:
>>> Hi
>>>
>>> I've been trying to make sense of some weird behavior with the seastar
>>> implementation of io_uring, and started to suspect a bug in io_uring's
>>> connect.
>>>
>>> The situation is as follows:
>>>
>>> - A connect() call is issued (and in the backend I can choose if I use
>>> uring or not)
>>> - The connection is supposed to take a while to establish.
>>> - I call shutdown on the file descriptor
>>>
>>> If io_uring is not used:
>>> - connect() starts by  returning EINPROGRESS as expected, and after
>>> the shutdown the file descriptor is finally made ready for epoll. I
>>> call getsockopt(SOL_SOCKET, SO_ERROR), and see the error (104)
>>>
>>> if io_uring is used:
>>> - if the SQE has the IOSQE_ASYNC flag on, connect() never returns.
>>> - if the SQE *does not* have the IOSQE_ASYNC flag on, then most of the
>>> time the test works as intended and connect() returns 104, but
>>> occasionally it hangs too. Note that, seastar may choose not to call
>>> io_uring_enter immediately and batch sqes.
>>>
>>> Sounds like some kind of race?
>>>
>>> I know C++ probably stinks like the devil for you guys, but if you are
>>> curious to see the code, this fails one of our unit tests:
>>>
>>> https://github.com/scylladb/seastar/blob/master/tests/unit/connect_test.cc
>>> See test_connection_attempt_is_shutdown
>>> (above is the master seastar tree, not including the io_uring implementation)
>>>
>> Is this chaining with connect().then_wrapped() asynchronous? Like kind
>> of future/promise stuff?
> Correct.
> then_wrapped executes eventually when connect returns either success or failure
>
>> I wonder, if connect() and shutdown() there may
>> be executed in the reverse order.
> The methods connect and shutdown will execute in this order.
> But connect will just queue something that will later be sent down to
> the kernel.
>
> I initially suspected an ordering issue on my side. What made me start
> suspecting a bug
> are two reasons:
> - I can force the code to grab an sqe and call io_uring_enter at the
> moment the connect()
> call happens : I see no change.
> - that IOSQE_ASYNC changes this behavior, as you acknowledged yourself.
>
> It seems to me that if shutdown happens when the sqe is sitting on a
> kernel queue somewhere
> the connection will hang forever instead of failing right away as I would expect
> - if shutdown happens after the call to io_uring_enter



You can try to cancel the sqe before you shutdown the socket. This will 
flush the queue (even if the cancellation fails).


However, if you io_uring_enter before calling shutdown and connect does 
not return, I'd consider that a kernel bug. Perhaps you can reduce the 
problem to a small C reproducer?