From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: oops in tcp_xmit_retransmit_queue() w/ v2.6.32.15 Date: Sun, 11 Jul 2010 19:46:22 +0200 Message-ID: <1278870382.2538.180.camel@edumazet-laptop> References: <4C358AAA.9080400@kernel.org> <1278867977.2538.167.camel@edumazet-laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Tejun Heo , "David S. Miller" , lkml , "netdev@vger.kernel.org" , "Fehrmann, Henning" , Carsten Aulbert To: Ilpo =?ISO-8859-1?Q?J=E4rvinen?= Return-path: In-Reply-To: <1278867977.2538.167.camel@edumazet-laptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Le dimanche 11 juillet 2010 =C3=A0 19:06 +0200, Eric Dumazet a =C3=A9cr= it : > Le dimanche 11 juillet 2010 =C3=A0 19:09 +0300, Ilpo J=C3=A4rvinen a = =C3=A9crit : > > On Thu, 8 Jul 2010, Tejun Heo wrote: > >=20 > > > We've been seeing oops in tcp_xmit_retransmit_queue() w/ 2.6.32.1= 5. > > > Please see the attached photoshoot. This is happening on a HPC > > > cluster and very interestingly caused by one particular job. How= long > > > it takes isn't clear yet (at least more than a day) but when it > > > happens it happens on a lot of machines in relatively short time. > > >=20 > > > With a bit of disassemblying, I've found that the oops is happeni= ng > > > during tcp_for_write_queue_from() because the skb->next points to > > > NULL. > > >=20 > > > void tcp_xmit_retransmit_queue(struct sock *sk) > > > { > > > ... > > > if (tp->retransmit_skb_hint) { > > > skb =3D tp->retransmit_skb_hint; > > > last_lost =3D TCP_SKB_CB(skb)->end_seq; > > > if (after(last_lost, tp->retransmit_high)) > > > last_lost =3D tp->retransmit_high; > > > } else { > > > skb =3D tcp_write_queue_head(sk); > > > last_lost =3D tp->snd_una; > > > } > > >=20 > > > =3D> tcp_for_write_queue_from(skb, sk) { > > > __u8 sacked =3D TCP_SKB_CB(skb)->sacked; > > >=20 > > > if (skb =3D=3D tcp_send_head(sk)) > > > break; > > > /* we could do better than to assign each time */ > > > if (hole =3D=3D NULL) > > >=20 > > > This can happen for one of the following reasons, > > >=20 > > > 1. tp->retransmit_skb_hint is NULL and tcp_write_queue_head() is = NULL > > > too. ie. tcp_xmit_retransmit_queue() is called on an empty wr= ite > > > queue for some reason. > > >=20 > > > 2. tp->retransmit_skb_hint is pointing to a skb which is not on t= he > > > write_queue. ie. somebody forgot to update hint while removin= g the > > > skb from the write queue. > >=20 > > Once again I've read the unlinkers through, and only thing that cou= ld=20 > > cause this is tcp_send_synack (others do deal with the hints) but I= think=20 > > Eric already proposed a patch to that but we never got anywhere due= to=20 > > some counterargument why it wouldn't take place (too far away for m= e to=20 > > remember, see archives about the discussions). ...But if you want b= e dead=20 > > sure some WARN_ON there might not hurt. Also the purging of the who= le=20 > > queue was a similar suspect I then came across (but that would only= =20 > > materialize with sk reuse happening e.g., with nfs which the other = guys=20 > > weren't using). > >=20 >=20 > Hmm. >=20 > This sounds familiar to me, but I cannot remember the discussion you > mention or the patch. >=20 > Or maybe it was the TCP transaction thing ? (including data in SYN or > SYN-ACK packet) Hmm, I cannot find where we reset restransmit_skb_hint in tcp_mtu_probe(), if we call tcp_unlink_write_queue(). if (skb->len <=3D copy) { /* We've eaten all the data from this skb. * Throw it away. */ TCP_SKB_CB(nskb)->flags |=3D TCP_SKB_CB(skb)->flags; <<>> tcp_unlink_write_queue(skb, sk); sk_wmem_free_skb(sk, skb); } else { Sorry if this was already discussed. We might add a comment here in any= case ;)