Messages in this thread | | | From | Oleksandr Natalenko <> | Subject | Re: [REGRESSION] tcp/ipv4: kernel panic because of (possible) division by zero | Date | Sun, 10 Jan 2016 23:56:54 +0200 |
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OK, it seems the assumption about YeAH is correct. Here is stacktrace fired by WARN_ON_ONCE():
https://gist.github.com/851cedcfca60d6120035
Is there sufficient info for you to prepare upstream patch?
On неділя, 10 січня 2016 р. 12:29:17 EET Neal Cardwell wrote: > On Sun, Jan 10, 2016 at 9:57 AM, Oleksandr Natalenko > > <oleksandr@natalenko.name> wrote: > > I use YeAH. But YeAH code wasn't touched between 4.2 and 4.3. > > Oh, interesting. Looks like tcp_yeah_ssthresh() has a bug where its > intended reduction can be bigger than tp->snd_cwnd, leading to it > return a zero ssthresh (or even an ssthresh that underflows to ~4 > billion). If tcp_yeah_ssthresh() returns an ssthresh of 0 then PRR > will try to pull the cwnd down to 0. > > Can you please leave ECN and Yeah enabled and run something like the > following patch, to verify this conjecture? If the conjecture is > right, then the tcp_yeah warning should fire but not the new > tcp_cwnd_reduction() warning: > > ----------- > diff --git a/net/ipv4/tcp_yeah.c b/net/ipv4/tcp_yeah.c > index 17d3566..ef60cba 100644 > --- a/net/ipv4/tcp_yeah.c > +++ b/net/ipv4/tcp_yeah.c > @@ -206,6 +206,7 @@ static u32 tcp_yeah_ssthresh(struct sock *sk) > const struct tcp_sock *tp = tcp_sk(sk); > struct yeah *yeah = inet_csk_ca(sk); > u32 reduction; > + s32 ssthresh; > > if (yeah->doing_reno_now < TCP_YEAH_RHO) { > reduction = yeah->lastQ; > @@ -219,7 +220,9 @@ static u32 tcp_yeah_ssthresh(struct sock *sk) > yeah->fast_count = 0; > yeah->reno_count = max(yeah->reno_count>>1, 2U); > > - return tp->snd_cwnd - reduction; > + ssthresh = tp->snd_cwnd - reduction; > + if (WARN_ON_ONCE(ssthresh <= 0)) > + ssthresh = 1; > } > > static struct tcp_congestion_ops tcp_yeah __read_mostly = { > ----------- > > If that works, then we may just want a version of this patch without > the warning. > > Thanks! > neal
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