linux-kernel - Re: [PATCH net] tcp: make TCP_USER_TIMEOUT accurate for zero window probes

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Date:   Sat, 23 Jan 2021 19:19:13 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Enke Chen <enkechen2020@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] tcp: make TCP_USER_TIMEOUT accurate for zero window probes

On Fri, Jan 22, 2021 at 9:45 PM Enke Chen <enkechen2020@...il.com> wrote:
>
> Hi, Jakub:
>
> On Fri, Jan 22, 2021 at 06:34:24PM -0800, Jakub Kicinski wrote:
> > On Fri, 22 Jan 2021 18:28:23 -0800 Enke Chen wrote:
> > > Hi, Jakub:
> > >
> > > In terms of backporting, this patch should go together with:
> > >
> > >     9d9b1ee0b2d1 tcp: fix TCP_USER_TIMEOUT with zero window
> >
> > As in it:
> >
> > Fixes: 9d9b1ee0b2d1 tcp: fix TCP_USER_TIMEOUT with zero window
> >
> > or does it further fix the same issue, so:
> >
> > Fixes: 9721e709fa68 ("tcp: simplify window probe aborting on USER_TIMEOUT")
> >
> > ?
>
> Let me clarify:
>
> 1) 9d9b1ee0b2d1 tcp: fix TCP_USER_TIMEOUT with zero window
>
>    fixes the bug and makes it work.
>
> 2) The current patch makes the TCP_USER_TIMEOUT accurate for 0-window probes.
>    It's independent.

Patch (2) ("tcp: make TCP_USER_TIMEOUT accurate for zero window
probes") is indeed conceptually independent of (1) but its
implementation depends on the icsk_probes_tstamp field defined in (1),
so AFAICT (2) cannot be backported further back than (1).

Patch (1) fixes a bug in 5.1:
    Fixes: 9721e709fa68 ("tcp: simplify window probe aborting on USER_TIMEOUT")

So probably (1) and (2) should be backported as a pair, and only back
as far as 5.1. (That covers 2 LTS kernels, 5.4 and 5.10, so hopefully
that is good enough.)

neal