Subject: Fw: [Bug 194723] New: connect() to localhost stalls after 4.9 -> 4.10 upgrade

From: Stephen Hemminger <[email-masked]>
Date: Mon, 27 Feb 2017 08:38:20 -0800

Begin forwarded message:

Date: Mon, 27 Feb 2017 11:28:51 +0000
From: [email-masked]
To: [email-masked]
Subject: [Bug 194723] New: connect() to localhost stalls after 4.9 -> 4.10 upgrade

https://bugzilla.kernel.org/show_bug.cgi?id=194723

            Bug ID: 194723
           Summary: connect() to localhost stalls after 4.9 -> 4.10
                    upgrade
           Product: Networking
           Version: 2.5
    Kernel Version: 4.10
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: IPV4
          Assignee: [email-masked]
          Reporter: [email-masked]
        Regression: No

After upgrading a machine running the latest CentOS from using mainline kernel
linux-4.9 to linux-4.10, attempts to connect() via IPv4 to localhost fail in
about half of the cases, leaving the process trying to connect() stalled.

Reproduction:

> ncat -k -l 19999 &
> C=1 ; while true ; do echo -n "$C " ; echo ping | ncat localhost 19999 ;
> C=`expr $C + 1` ; sleep 1 ; done  

Using linux-4.10, the output looks like this:

> 1 ping
> 2 Ncat: Connection timed out.
> 3 ping
> 4 Ncat: Connection timed out.
> 5 ping
> 6 ping
> 7 ping
> 8 Ncat: Connection timed out.
> 9 ping
> 10 ping
> 11 Ncat: Connection timed out.
> 12 ping
> 13 Ncat: Connection timed out.
> 14 ping
> 15 Ncat: Connection timed out.
> 16 ping
> 17 Ncat: Connection timed out.
> 18 ping
> 19 ping
> 20 Ncat: Connection timed out.
> 21 ping
> 22 ping
> 23 Ncat: Connection timed out.
> 24 ping
> 25 Ncat: Connection timed out.
> 26 Ncat: Connection timed out.
> 27 ping
> 28 Ncat: Connection timed out.
> 29 ping  

Using linux-4.9, the output looks like this:

> 1 ping
> 2 ping
> 3 ping
> 4 ping
> 5 ping
> 6 ping
> 7 ping
> 8 ping
> 9 ping
> 10 ping
> 11 ping
> 12 ping
> 13 ping
> 14 ping
> 15 ping
> 16 ping
> 17 ping
> 18 ping
> 19 ping  

(The same behaviour was later also confirmed by a colleague on an Ubuntu
running machine after upgrading to Ubuntu's 4.10 kernel.)

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