netdev - Re: [REGRESSION] Select hang with zero sized UDP packets

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Message-Id: <20160823.112515.318902967155957764.davem@davemloft.net>
Date:   Tue, 23 Aug 2016 11:25:15 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     labbott@...hat.com
Cc:     kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
        kaber@...sh.net, samanthakumar@...gle.com, willemb@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] Select hang with zero sized UDP packets

From: Laura Abbott <labbott@...hat.com>
Date: Tue, 23 Aug 2016 10:53:26 -0700

> Fedora received a report[1] of a unit test failing on Ruby when using
> the
> 4.7 kernel. This was a test to send a zero sized UDP packet. With the
> 4.7 kernel, the test now timing out on a select instead of completing.
> The reduced ruby test is
> 
>   def test_udp_recvfrom_nonblock
>     u1 = UDPSocket.new
>     u2 = UDPSocket.new
>     u1.bind("127.0.0.1", 0)
>     u2.send("", 0, u1.getsockname)
>     IO.select [u1]  # test gets stuck here
>   ensure
>     u1.close if u1
>     u2.close if u2
>   end

Well, if there is no data, should select really wake up?

I think it's valid not to.