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Date:   Sat, 23 Jan 2021 19:36:24 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Enke Chen <enkechen2020@gmail.com>,
        Neal Cardwell <ncardwell@google.com>,
        Eric Dumazet <edumazet@google.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] tcp: make TCP_USER_TIMEOUT accurate for zero window
 probes
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On Sat, 23 Jan 2021 16:56:43 -0800 Enke Chen wrote:
> On Sat, Jan 23, 2021 at 07:19:13PM -0500, Neal Cardwell wrote:
> > On Fri, Jan 22, 2021 at 9:45 PM Enke Chen <enkechen2020@gmail.com> wrote:  
> > > 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:  
> > > > > 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.)
> 
> What you described is more accurate, and is correct.

That makes it clear.

I added a Fixes tag, reworded the message slightly and applied, thanks!