linux-kernel - Re: disappearing listen()ed SO_REUSEPORT sockets across fork() when using epoll

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Message-ID: <CAJusiZVjiH6P4cLZULrS4KWk8WUQUmG61zrJJNM5L_KzOYKh-Q@mail.gmail.com>
Date:	Mon, 25 Nov 2013 13:03:46 -0800
From:	Shawn Landden <shawnlandden@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Jason Baron <jbaron@...mai.com>, linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: disappearing listen()ed SO_REUSEPORT sockets across fork() when
 using epoll

On Mon, Nov 25, 2013 at 12:05 PM, Mateusz Guzik <mguzik@...hat.com> wrote:
> On Mon, Nov 25, 2013 at 11:53:24AM -0800, Shawn Landden wrote:
>> On Mon, Nov 25, 2013 at 10:05 AM, Jason Baron <jbaron@...mai.com> wrote:
>> > On 11/22/2013 12:53 PM, Shawn Landden wrote:
>> >> Hello, when running the attached program on 3.12 child processes
>> >> are missing a socket fd opened, set with SO_REUSEPORT, listen()ed to,
>> >> and added to epoll_ctl().
>> >>
>> >> This is the output I get when pointing "wget http://localhost:5555/"
>> >> at the attached program:
>> >>
>> >> main PID 31591
>> >> PID 31634 started
>> >> PID 31634 accept()ed connection
>> >> PID 31635 started
>> >> PID 31636 started
>> >> PID 31635 accept() failed: Bad file descriptor
>> >> PID 31636 accept() failed: Bad file descriptor
>> >> PID 31634 accept()ed connection
>> >> PID 31634 accept()ed connection
>> >> PID 31634 accept()ed connection
>> >> PID 31634 accept()ed connection
>> >>
>> >>
>> >> While I would expect something like:
>> >>
>> >> main PID 31591
>> >> PID 31634 started
>> >> PID 31634 accept()ed connection
>> >> PID 31635 started
>> >> PID 31636 started
>> >> PID 31635 accept()ed connection
>> >> PID 31636 accept()ed connection
>> >>
>> >> -more new processes, but inversely proportional to number of listening processes
>> >> -accept() always returns successfully
>> >>
>> >>
>> >
>> > The 'close(sockfd);' looks to be racing with the accept() calls. Removing seems
>> > to get the result you are looking for.
>> Interesting. That works, but it shouldn't. The close() is operating in
>> the parent, so it shouldn't affect the child,
>> there is a leak here of process separation.
>>
>
> You fork, then close sockfd in the parent. Thus, the very first child
> can accept connectins just fine.
>
> Subsequent forks give you children without sockfd, thus accept fails.
> The first child continues to work just fine.
Now I feel like an idiot.



-- 

---
Shawn Landden
+1 360 389 3001 (SMS preferred)
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