From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Friesen Subject: Re: weird behaviour, getting EAGAIN on a connect() call on a unix stream socket Date: Sat, 2 Aug 2014 08:11:43 -0600 Message-ID: <53DCF19F.5030002@windriver.com> References: <53DC6039.2010002@windriver.com> <1406960889.3178.60.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Cc: To: Eric Dumazet Return-path: Received: from mail1.windriver.com ([147.11.146.13]:49223 "EHLO mail1.windriver.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932114AbaHBOLt (ORCPT ); Sat, 2 Aug 2014 10:11:49 -0400 In-Reply-To: <1406960889.3178.60.camel@edumazet-glaptop2.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: On 08/02/2014 12:28 AM, Eric Dumazet wrote: > On Fri, 2014-08-01 at 21:51 -0600, Chris Friesen wrote: >> I've got an app that tries to connect() to both of them in turn. The connect() >> to the first socket fails with EAGAIN, the second one succeeds, and all >> subsequent retries on the first fail. Here's an strace() of the sequence: >> >> socket(PF_FILE, SOCK_STREAM, 0) = 6 >> fcntl(6, F_GETFL) = 0x2 (flags O_RDWR) >> fcntl(6, F_SETFL, O_RDWR|O_NONBLOCK) = 0 > > Non blocking socket : If listener queue is full, -EAGAIN is expected That doesn't make any sense though, there is only one process that ever attempts to connect() to this socket, and I only ran it one instance at a time. That implies that the first time I got EAGAIN the queue would have been empty when the connection request came in. >> With the app not running, netstat seems to show that something is trying to >> connect to the socket in question: >> >> root@compute-0:~# netstat -ap unix |grep messaging >> unix 2 [ ACC ] STREAM LISTENING 1109818 17379/qemu-system-x /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> unix 2 [ ACC ] STREAM LISTENING 1110051 17425/qemu-system-x /var/lib/libvirt/qemu/cgcs.messaging.instance-00000008.sock >> unix 2 [ ] STREAM CONNECTING 0 - /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> unix 2 [ ] STREAM CONNECTING 0 - /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> unix 2 [ ] STREAM CONNECTED 1109848 17379/qemu-system-x /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> >> >> Here's /proc/net/unix for completeness: >> >> root@compute-0:~/host-guest-comm# grep -a messaging /proc/net/unix >> ffff880045c35540: 00000002 00000000 00010000 0001 01 1109818 /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> ffff8800576b8a80: 00000002 00000000 00010000 0001 01 1110051 /var/lib/libvirt/qemu/cgcs.messaging.instance-00000008.sock >> ffff880045e2f040: 00000002 00000000 00000000 0001 02 0 /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> ffff88004bc5ea80: 00000002 00000000 00000000 0001 02 0 /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> ffff880045e2f540: 00000002 00000000 00000000 0001 03 1109848 /var/lib/libvirt/qemu/cgcs.messaging.instance-00000007.sock >> >> >> >> The crazy thing is that I can't figure out what could be causing the >> CONNECTED/CONNECTING sockets. There are no background processes of the >> connecting app running, no zombie processes, no forked children, etc. >> >> Just to make things more interesting, I successfully ran this application >> several times (connecting to both sockets) before this behaviour started >> happening. I was running it under strace and just killed it with ctrl-C. >> >> Anyone got any ideas? Please CC me since I'm not subscribed to the list. > > The application might use a too small listen() backlog ? Looking at the qemu code I think it's calling listen(sock,1) which makes sense since I think it's only designed to allow a single connection up into the guest at a time. Not sure how that could be the problem though, since there is only one process that tries to connect() to the application, and I only ran it one instance at a time. I'll give the patch a try, but how would that explain the sockets that are in a CONNECTING state when as far as I can tell they don't belong to any process? Am I correct to think that the CONNECTED socket may be due to the two CONNECTING ones somehow? Chris