Date: Tue, 9 Apr 2013 20:44:39 +0200 From: Peter Holm <peter@holm.cc> To: Andre Oppermann <andre@freebsd.org> Cc: FreeBSD Net <freebsd-net@freebsd.org>, Juan Mojica <jmojica@gmail.com>, Matt Miller <matt@matthewjmiller.net>, Rick Macklem <rmacklem@uoguelph.ca> Subject: Re: panic in tcp_do_segment() Message-ID: <20130409184439.GA62925@x2.osted.lan> In-Reply-To: <5163D2D2.2090407@freebsd.org> References: <1043692819.529554.1365114790772.JavaMail.root@erie.cs.uoguelph.ca> <CAFc6gu9bmJQ-Uxv7PzspSLDyFz6zE3fV=u7HEumCh6f=YQLU2Q@mail.gmail.com> <5162B474.6060808@freebsd.org> <20130409081652.GA50498@x2.osted.lan> <5163D2D2.2090407@freebsd.org>
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On Tue, Apr 09, 2013 at 10:35:30AM +0200, Andre Oppermann wrote: > On 09.04.2013 10:16, Peter Holm wrote: > > On Mon, Apr 08, 2013 at 02:13:40PM +0200, Andre Oppermann wrote: > >> On 05.04.2013 13:09, Matt Miller wrote: > >>> Hey Rick, > >>> > >>> I believe Juan and I have root caused this crash recently. The t_state = > >>> 0x1, TCPS_LISTEN, in the link provided at the time of the assertion. > >>> > >>> In tcp_input(), if we're in TCPS_LISTEN, SO_ACCEPTCONN should be set on the > >>> socket and we should never enter tcp_do_segment() for this state. I think > >>> if you look in your corefile, you'll see the socket *doesn't* have this > >>> flag set in your case. > >>> > >>> 1043 /* > >>> 1044 * When the socket is accepting connections (the INPCB is in > >>> LISTEN > >>> 1045 * state) we look into the SYN cache if this is a new > >>> connection > >>> 1046 * attempt or the completion of a previous one. Because listen > >>> 1047 * sockets are never in TCPS_ESTABLISHED, the V_tcbinfo lock > >>> will be > >>> 1048 * held in this case. > >>> 1049 */ > >>> 1050 if (so->so_options & SO_ACCEPTCONN) { > >>> 1051 struct in_conninfo inc; > >>> 1052 > >>> 1053 KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so accepting > >>> but " > >>> 1054 "tp not listening", __func__)); > >>> ... > >>> 1356 syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL); > >>> 1357 /* > >>> 1358 * Entry added to syncache and mbuf consumed. > >>> 1359 * Everything already unlocked by syncache_add(). > >>> 1360 */ > >>> 1361 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo); > >>> 1362 return; > >>> 1363 } > >>> ... > >>> 1384 /* > >>> 1385 * Segment belongs to a connection in SYN_SENT, ESTABLISHED or > >>> later > >>> 1386 * state. tcp_do_segment() always consumes the mbuf chain, > >>> unlocks > >>> 1387 * the inpcb, and unlocks pcbinfo. > >>> 1388 */ > >>> 1389 tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos, > >>> ti_locked); > >>> > >>> I think this has to do with this patch in soclose() where SO_ACCEPTCONN is > >>> being turned off in soclose(). I suspect if you look at the other threads > >>> in your corefile, you'll see one at this point in soclose() operating on > >>> the same socket as the one in the tcp_do_segment() thread. > >>> > >>> http://svnweb.freebsd.org/base?view=revision&revision=243627 > >>> > >>> 817 /* > >>> 818 * Prevent new additions to the accept queues due > >>> 819 * to ACCEPT_LOCK races while we are draining them. > >>> 820 */ > >>> 821 so->so_options &= ~SO_ACCEPTCONN; > >>> 822 while ((sp = TAILQ_FIRST(&so->so_incomp)) != NULL) { > >>> 823 TAILQ_REMOVE(&so->so_incomp, sp, so_list); > >>> 824 so->so_incqlen--; > >>> 825 sp->so_qstate &= ~SQ_INCOMP; > >>> 826 sp->so_head = NULL; > >>> 827 ACCEPT_UNLOCK(); > >>> 828 soabort(sp); > >>> 829 ACCEPT_LOCK(); > >>> 830 } > >>> > >>> Juan had evaluated this code path and it seemed safe to just drop the > >>> packet in this case: > >>> > >>> + /* > >>> + * In closing down the socket, the SO_ACCEPTCONN flag is removed to > >>> + * prevent new connections from being established. This means that > >>> + * any frames in that were in the midst of being processed could > >>> + * make it here. Need to just drop the frame. > >>> + */ > >>> + if (TCPS_LISTEN == tp->t_state) { > >>> + TCPSTAT_INC(tcps_rcvwhileclosing); > >>> + goto drop; > >>> + } > >>> KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN", > >>> __func__)); > >>> > >>> Or, if there's someone more familiar with the locking in these paths, they > >>> may be able to come up with a way to restructure the locks and logic to > >>> close this window. > >> > >> Matt, Juan, > >> > >> excellent analysis. I don't see a better approach to handle this > >> under the current ACCEPT_LOCK model. > >> > >> Compared to your patch I'd like to handle this race earlier before > >> we hit tcp_do_segment(). > >> > >> Could you please review the attached patch which handles it right > >> after the SO_ACCEPTCONN / syncache check? > >> > >> -- > >> Andre > >> > >> Index: netinet/tcp_input.c > >> =================================================================== > >> --- netinet/tcp_input.c (revision 249253) > >> +++ netinet/tcp_input.c (working copy) > >> @@ -1351,6 +1351,16 @@ > >> */ > >> INP_INFO_UNLOCK_ASSERT(&V_tcbinfo); > >> return; > >> + } else if (tp->t_state == TCPS_LISTEN) { > >> + /* > >> + * When a listen socket is torn down the SO_ACCEPTCONN > >> + * flag is removed first while connections are drained > >> + * from the accept queue in a unlock/lock cycle of the > >> + * ACCEPT_LOCK, opening a race condition allowing a SYN > >> + * attempt go through unhandled. > >> + */ > >> + TCPSTAT_INC(tcps_rcvdwhileclosing); > >> + goto drop; > >> } > >> > >> #ifdef TCP_SIGNATURE > > > > I was able to reproduce the original "panic: tcp_do_segment: > > TCPS_LISTEN" with ease; see > > http://people.freebsd.org/~pho/stress/log/tcp.txt. > > > > With your patch (minus the TCPSTAT_INC) I got this "panic: Lock (rw) > > tcp locked @ netinet/tcp_input.c:1432." > > > > http://people.freebsd.org/~pho/stress/log/tcp2.txt > > Please replace the 'goto drop' with 'goto dropunlock' to fix the panic. > Yes, this seems to take care of the two problems reported. Tested for 5 hours without any repeats. - Peter
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