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From: Avi Kivity <avi@scylladb.com>
To: Glauber Costa <glauber@scylladb.com>,
	Pavel Begunkov <asml.silence@gmail.com>
Cc: io-uring@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: shutdown not affecting connection?
Date: Sat, 8 Feb 2020 20:48:50 +0200
Message-ID: <9ec6cbf7-0f0b-f777-8507-199e8837df94@scylladb.com> (raw)
In-Reply-To: <CAD-J=zbMcPx1Q5PTOK2VTBNVA+PQX1DrYhXvVRa2tPRXd_2RYQ@mail.gmail.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?



  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-08 13:55 Glauber Costa
2020-02-08 14:26 ` Pavel Begunkov
2020-02-08 18:42   ` Glauber Costa
2020-02-08 18:48     ` Avi Kivity [this message]
2020-02-08 18:57       ` Glauber Costa
2020-02-08 20:20         ` Glauber Costa
2020-02-08 20:28           ` Avi Kivity
2020-02-08 20:43             ` Glauber Costa
2020-02-08 18:48 ` Andres Freund
2020-02-08 18:54   ` Glauber Costa

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