From: Avi Kivity <email@example.com> To: Glauber Costa <firstname.lastname@example.org>, Pavel Begunkov <email@example.com> Cc: firstname.lastname@example.org, Jens Axboe <email@example.com> Subject: Re: shutdown not affecting connection? Date: Sat, 8 Feb 2020 20:48:50 +0200 Message-ID: <firstname.lastname@example.org> (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 <email@example.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?
next prev parent 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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