From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: a problem tcp_v4_err()
Date: Fri, 12 Nov 2010 19:27:57 +0100
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Cc: Patrick McHardy <kaber@trash.net>,
	David Lamparter <equinox@diac24.net>,
	Eric Paris <eparis@redhat.com>, Hua Zhong <hzhong@gmail.com>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	davem@davemloft.net, pekkas@netcore.fi, jmorris@namei.org,
	yoshfuji@linux-ipv6.org, paul.moore@hp.com,
	Damian Lukowski <damian@tvk.rwth-aachen.de>
To: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Return-path: <linux-kernel-owner@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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Le vendredi 12 novembre 2010 =C3=A0 19:21 +0100, Eric Dumazet a =C3=A9c=
rit :
> Le vendredi 12 novembre 2010 =C3=A0 19:12 +0100, Eric Dumazet a =C3=A9=
crit :
> > Le vendredi 12 novembre 2010 =C3=A0 20:57 +0300, Alexey Kuznetsov a=
 =C3=A9crit :
> > > Hello!
> > >=20
> > > I looked at tcp_v4_err() and found something strange. Quite non-t=
rivial operations
> > > are performed on unlocked sockets. It looks like at least this BU=
G_ON():
> > >=20
> > >                 skb =3D tcp_write_queue_head(sk);
> > >                 BUG_ON(!skb);
> > >=20
> > > can be easily triggered.
> > >=20
> > > Do I miss something?
> > >=20
> >=20
> > Hi Alexey !
> >=20
> > I see socket is locked around line 368,
> >=20
> >         bh_lock_sock(sk);
> >         /* If too many ICMPs get dropped on busy
> >          * servers this needs to be solved differently.
> >          */
> >         if (sock_owned_by_user(sk))
> >                 NET_INC_STATS_BH(net, LINUX_MIB_LOCKDROPPEDICMPS);
> >=20
> >=20
> > Hmm, maybe some goto is missing ;)
> >=20
>=20
> Well, goto is not missing.
>=20
> Why do you think BUG_ON(!skb) can be triggered ?
>=20
> We test before :
>=20
> 	if (seq !=3D tp->snd_una  || !icsk->icsk_retransmits ||
> 		!icsk->icsk_backoff)
> 		break;
>=20
> So a concurrent user only can add new skb(s) in the (non empty) queue=
 ?
>=20
>=20

Oh well, it seems you are right (backlog processing)

Bug was introduced in commit f1ecd5d9e736660 (Revert Backoff [v3]:
Revert RTO on ICMP destination unreachable) from Damian Lukowski