netdev - Re: [PATCH stable-5.4 4/4] tcp: avoid too many retransmit packets

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Message-ID: <2024071610-cascade-recall-ef1f@gregkh>
Date: Tue, 16 Jul 2024 15:03:12 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, edumazet@...gle.com,
	davem@...emloft.net, eric.dumazet@...il.com, jmaxwell37@...il.com,
	kuba@...nel.org, kuniyu@...zon.com, ncardwell@...gle.com,
	netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH stable-5.4 4/4] tcp: avoid too many retransmit packets

On Tue, Jul 16, 2024 at 02:56:28PM +0200, Greg KH wrote:
> On Tue, Jul 16, 2024 at 02:53:12PM +0200, Greg KH wrote:
> > On Tue, Jul 16, 2024 at 08:40:40PM +0800, Jason Xing wrote:
> > > On Tue, Jul 16, 2024 at 7:10 PM Miguel Ojeda <ojeda@...nel.org> wrote:
> > > >
> > > > Hi Greg, Eric, all,
> > > >
> > > > I noticed this in stable-rc/queue and stable-rc/linux- for 6.1 and 6.6:
> > > >
> > > >     net/ipv4/tcp_timer.c:472:7: error: variable 'rtx_delta' is uninitialized when used here [-Werror,-Wuninitialized]
> > > >                     if (rtx_delta > user_timeout)
> > > >                         ^~~~~~~~~
> > > >     net/ipv4/tcp_timer.c:464:15: note: initialize the variable 'rtx_delta' to silence this warning
> > > >             u32 rtx_delta;
> > > >                         ^
> > > >                         = 0
> > > >
> > > > I hope that helps!
> > > 
> > > Thanks for the report!
> > > 
> > > I think it missed one small snippet of code from [1] compared to the
> > > latest kernel. We can init this part before using it, something like
> > > this:
> > > 
> > > +       rtx_delta = (u32)msecs_to_jiffies(tcp_time_stamp(tp) -
> > > +                       (tp->retrans_stamp ?: tcp_skb_timestamp(skb)));
> > > 
> > > Note: fully untested.
> > > 
> > > Since Eric is very busy, I decided to check and provide some useful
> > > information here.
> > 
> > Thanks all, this was probably due to my manual backporting here, let me
> > go check what went wrong...
> 
> Yeah, this is my fault, due to 614e8316aa4c ("tcp: add support for usec
> resolution in TCP TS values") not being in the tree, let me go rework
> things...

Ok, backporting that commit is not going to happen, that's crazy...

Anyway, the diff below is what I made on top of the existing one, which
should be doing the right thing.  But ideally someone can test this,
somehow...  I'll push out -rc releases later today so that people can
pound on it easier.

thanks for the review!

greg k-h


--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -464,6 +464,9 @@ static bool tcp_rtx_probe0_timed_out(con
 	u32 rtx_delta;
 	s32 rcv_delta;
 
+	rtx_delta = (u32)msecs_to_jiffies(tcp_time_stamp(tp) -
+			(tp->retrans_stamp ?: tcp_skb_timestamp(skb)));
+
 	if (user_timeout) {
 		/* If user application specified a TCP_USER_TIMEOUT,
 		 * it does not want win 0 packets to 'reset the timer'
@@ -482,9 +485,6 @@ static bool tcp_rtx_probe0_timed_out(con
 	if (rcv_delta <= timeout)
 		return false;
 
-	rtx_delta = (u32)msecs_to_jiffies(tcp_time_stamp(tp) -
-			(tp->retrans_stamp ?: tcp_skb_timestamp(skb)));
-
 	return rtx_delta > timeout;
 }