On Thu, Jun 16, 2016 at 1:40 PM, Kirill Smelkov <k...@nexedi.com> wrote: > On Thu, Jun 16, 2016 at 08:38:49AM -0700, Dave Taht wrote: >> On Thu, Jun 16, 2016 at 4:17 AM, Kirill Smelkov <k...@nexedi.com> wrote: >> > On Wed, Jun 15, 2016 at 12:56:34PM +0200, Juliusz Chroboczek wrote: >> >> >> If I read you correctly, this looks like a kernel bug: incorrect >> >> >> invalidation of the route cache. >> >> >> >> [...] >> >> >> >> > What we have here is of another kind - it is inherent race condition >> >> > inside kernel >> >> >> >> Perhaps I'm confused, but it still looks like a kernel bug to me. >> > >> > Yes, it is a kernel bug. But in a sense it is so old and so widespread >> > that it has to be cared about in userspace - as with atomic route >> > updates we do not hit it. >> > >> > Also: atomic route updates are needed not only for avoiding this bug. >> > Another reason is: if we have routedel & routeadd pair, even after >> > routeadd the state of cache is correct, in the time between del & add, >> > if a packet destined to that route gets to the node, it hits >> > 'unreachable' route case. >> > >> > For usual packets it is only "packet lost" and TCP probably retransmits. >> > But for SYN packets, e.g. when a connection is going to be established, >> > ICMP error is returned which results in "host unreachable" error on >> > originator side. >> >> Yes this variant of the bug is still there, essentially, and it bugs me. >> >> (btw the facebook page you pointed to fixes they did was fascinating - >> they have "interesting problems" - like dealing with 1+m routes in >> their route table) >> >> one day a year, for several years now, I get sufficiently irked about >> the atomic update problem in babel to refresh my knowledge of netlink, >> hack babel all to hell, and have nothing work. I left myself a bunch >> more breadcrumbs last night in my hacked up babel version, as to what >> I tried and what it did wrong... (because I'm actually also chasing >> another bug which I'll put up in another message).... >> >> But: >> >> Why doing the equivalent of this (and understanding how it does it) >> >> ip -6 route add fd99::33/128 via fe80::120d:7fff:fe64:c992 dev eno1 >> ip -6 route replace fd99::33/128 via fe80::120d:7fff:fe64:c991 dev wlp2s0 >> >> is so hard for me to figure out - that I don't understand. But it >> seems to require completely tracing through the ip route code, and >> writing a decoder for the netlink packets created, to figure out why >> what I thought would be an equivalent for babel, and taking the week >> or more to do it... >> >> -- look! Squirrel! > > Dave, maybe this might help you: Wireshark (not tcpdump) has decoder for > netlink route packets: > > https://code.wireshark.org/review/gitweb?p=wireshark.git;a=blob;f=epan/dissectors/packet-netlink-route.c;hb=v2.1.1rc0-170-gc269684
Groovy. Thank you. I did not know. In discussing this with shemminger this morning, he pointed out there was a semantic difference between how routes can be replaced in ipv6 and ipv4. At *one point* last night I thought I'd successfully got ipv6 to atomic replace, but it had failed on ipv4 - so I will revisit the work soon, brain cells and time willing. > so you can create a virtual netlink monitor interface - something along > the lines of > > modprobe nlmon > ip link add type nlmon > ip link set nlmon0 up > > ( see more details in e.g. https://patchwork.ozlabs.org/patch/259444/ ) > > and see the actual packets exchanged between iproute and kernel. > > Also: there is pyroute2 (https://github.com/svinota/pyroute2) which has debug > decoder for netlink packets, but out of the box you have to specify packet > type > explicitly: > > https://github.com/svinota/pyroute2/blob/master/docs/debug.rst > > Maybe you already know all this, but I decided to provide info anyway to make > sure it is not missed, because you mentioned it is hard for you to understand > what is going on underneath `ip -6 ...` > > Hope this might help, > Kirill > > >> >> Perhaps it would make sense to speak to netdev about that? >> > >> > Yes, makes sense. Though as this particular case is not present on 4.2+ >> > kernels, people on netdev will probably has less interest to look into. >> > >> > I will see what can be done. >> > >> >> > Quagga, at least, switched to atomic updates some time ago, I think. >> >> > >> >> > http://patchwork.quagga.net/patch/1234/ >> >> >> >> I see. I'm busy right now, but I'll be grateful for a patch. >> > >> > I see about this. Thanks for feedback. >> > >> > >> > On Wed, Jun 15, 2016 at 07:35:05PM -0700, Dave Taht wrote: >> >> > https://lab.nexedi.com/kirr/iproute2/blob/bd480e66/t/rtcache-torture >> >> > (also attached to this email) >> >> > >> >> > which reproduces the problem in several minutes just on one computer and >> >> > retested it locally: I can reliably reproduce the issue on pristine >> >> > Debian 3.16.7-ckt25-2 (on both Atom and Core2 notebooks) and on pristine >> >> > 3.16.35 on Atom (compiled by me, since Debian kernel team has not yet >> >> > uploaded 3.16.35 to Jessie). >> >> >> >> I have been running this script on four different machines for hours >> >> now without reproducing your bug on the 4.4 or later kernels. It does >> >> trigger on a 3.14 kernel. (it helps to do a killall fping6 before >> >> exiting!) >> >> >> >> It does not seem to be happening on 4.4 or later. At one level, I'm >> >> relieved - one last babel bug to worry about in openwrt (now 4.4 >> >> based), although one of the platforms I work on is still stuck at >> >> 3.18, as is the 3.14 c2 (for now). >> >> >> >> At another level I still really, really, really wanted atomic updates >> >> in babel, and was clearing the decks to make a run at the right >> >> netlink stuff when I'd decided to confirm your bug existed or not in >> >> my kernels. :(. Weirdly demotivating. >> >> >> >> >> >> d@dancer:~/bin$ ssh root@pi3 uname -a >> >> Linux pi3 4.4.12-v7+ #892 SMP Thu Jun 2 15:41:19 BST 2016 armv7l GNU/Linux >> >> d@dancer:~/bin$ ssh root@pi2 uname -a >> >> Linux pi2 4.4.12-v7+ #892 SMP Thu Jun 2 15:41:19 BST 2016 armv7l GNU/Linux >> >> d@dancer:~/bin$ uname -a >> >> Linux dancer 4.5.0-rc7-fqfi #1 SMP PREEMPT Mon Mar 7 16:04:17 PST 2016 >> >> x86_64 x86_64 x86_64 GNU/Linux >> >> >> >> ... >> >> >> >> The odroid C2 has the bug. >> >> >> >> d@dancer:~/bin$ ssh root@c2 uname -a >> >> Linux c2 3.14.29-56 #1 SMP PREEMPT Wed Apr 20 12:15:54 BRT 2016 >> >> aarch64 aarch64 aarch64 GNU/Linux >> >> >> >> BUG: Got unexpected unreachable route for 2226:3333:4444:5555::1: # >> >> I'd changed the number >> >> unreachable 2226:3333:4444:5555::1 from :: dev lo src fd99::2 metric >> >> 0 \ cache error -101 >> >> >> >> route table for root 2226:3333:4444::/48 >> >> ---- 8< ---- >> >> unicast 2226:3333:4444:5555::/64 dev dum0 proto boot scope global >> >> metric 1024 >> >> unreachable 2226:3333:4444::/48 dev lo proto boot scope global >> >> metric 1024 error -101 >> >> ---- 8< ---- >> >> >> >> route for 2226:3333:4444:5555::1 (once again) >> >> unreachable 2226:3333:4444:5555::1 from :: dev lo src fd99::2 metric >> >> 0 \ cache error -101 users 1 used 3 >> > >> > Dave, thanks for confirming and for feedback about this. >> > >> > Yes, 4.2+ kernels should not have this _particular_ bug, because >> > https://git.kernel.org/linus/45e4fd26 reworks ip6_pol_route() for above >> > tested case to not lock the route table twice and not to create /128 >> > cache entries on lookup when there is a gateway. >> > >> > BUT >> > >> > Route cache for IPv6 is still there in new kernels, and sometimes cache >> > entries are created. E.g. this happens on PMTU exception, but also for >> > lookups without gateway when associated flow has FLOWI_FLAG_KNOWN_NH set >> > (I don't yet know what it is yet, but still): >> > >> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/route.c?id=v4.7-rc3-55-gd325ea8#n1089 >> > >> > etc. >> > >> > So _related_ problems should be there. They are probably just maybe less >> > easily reproducible and less often happening. I have not looked into >> > further details though... >> > >> > And also: as shown above it is better to have atomic route updates even >> > without cache issues to get SYN not occasionally rejected in the time of >> > route update. >> > >> > So Dave, please keep up your motivation for fixing this if you were >> > going to eventually do so. >> > >> > Thanks, >> > Kirill >> > >> > P.S. >> > >> >> (it helps to do a killall fping6 before exiting!) >> > >> > There is >> > >> > trap 'kill $(jobs -p)' EXIT >> > >> > it does not work? >> > >> > >> >> > It is always the same: the issue reproduces reliably in several minutes. >> >> > And it looks like e.g. >> >> > >> >> > ----- 8< ---- >> >> > root@mini:/home/kirr/src/tools/net/iproute2/t# time >> >> > ./rtcache-torture >> >> > PING 2222:3333:4444:5555::1(2222:3333:4444:5555::1) 56 data bytes >> >> > E.E.E.....E......E..E............E...E.. >> >> > <more output from ping> >> >> > >> >> > BUG: Linux mini 3.16.35-mini64 #14 SMP PREEMPT Sun Jun 12 19:41:09 >> >> > MSK 2016 x86_64 GNU/Linux >> >> > BUG: Got unexpected unreachable route for 2222:3333:4444:5555::1: >> >> > unreachable 2222:3333:4444:5555::1 from :: dev lo src >> >> > 2001:67c:1254:20::1 metric 0 \ cache error -101 >> >> > >> >> > route table for root 2222:3333:4444::/48 >> >> > ---- 8< ---- >> >> > unicast 2222:3333:4444:5555::/64 dev dum0 proto boot scope >> >> > global metric 1024 >> >> > unreachable 2222:3333:4444::/48 dev lo proto boot scope global >> >> > metric 1024 error -101 >> >> > ---- 8< ---- >> >> > >> >> > route for 2222:3333:4444:5555::1 (once again) >> >> > unreachable 2222:3333:4444:5555::1 from :: dev lo src >> >> > 2001:67c:1254:20::1 metric 0 \ cache error -101 users 1 used 4 >> >> > >> >> > real 0m49.938s >> >> > user 0m4.488s >> >> > sys 0m5.872s >> >> > ---- 8< ---- >> >> > >> >> > The issue should not show itself with kernels >= 4.2, because there the >> >> > lookup procedure does not take table lock twice, and /128 cache entries >> >> > are not routinely created (they are created only upon PMTU exception). >> >> > >> >> > I'm running Debian testing on my development machine. Currently it has >> >> > 4.5.5-1 (2016-05-29). I can confirm that /128 route cache entries are >> >> > not created there just because a route was looked up. >> >> > >> >> > Kirill >> >> > >> >> > >> >> > ---- 8< ---- (rtcache-torture) >> >> > #!/bin/sh -e >> >> > # torture for IPv6 RT cache, trying to hit the race between >> >> > lookup,cache-add & route add >> >> > # >> >> > http://lists.alioth.debian.org/pipermail/babel-users/2016-June/002547.html >> >> > >> >> > >> >> > tprefix=2222:3333:4444 # "whole-network" prefix for tests /48 >> >> > tsubnet=$tprefix:5555 # subnetwork for which "to" route will be >> >> > changed /64 >> >> > taddr=$tsubnet::1 # test address on $tsubnet >> >> > >> >> > # setup for tests: >> >> > >> >> > # dum0 dummy device >> >> > ip link del dev dum0 2>/dev/null || : >> >> > ip link add dum0 type dummy >> >> > ip link set up dev dum0 >> >> > >> >> > # clean route table for tprefix with only unreachable whole-network >> >> > route >> >> > ip -6 route flush root $tprefix::/48 >> >> > ip -6 route add unreachable $tprefix::/48 >> >> > ip -6 route flush cache >> >> > >> >> > ip -6 route add $tsubnet::/64 dev dum0 >> >> > >> >> > >> >> > # put a lot of requests to rt/rtcache getting route to $taddr >> >> > trap 'kill $(jobs -p)' EXIT >> >> > rtgetter() { >> >> > # NOTE we cannot do this with `ip route get ...` in a loop, as `ip >> >> > route >> >> > # get` first takes RTNL lock, and thus will be completely >> >> > serialized with >> >> > # e.g. route add and del. >> >> > # >> >> > # Ping, like other usually connect/tx activity works without RTNL >> >> > held. >> >> > exec ping6 -n -f $taddr >> >> > } >> >> > rtgetter & >> >> > >> >> > # do route del/route in busyloop; >> >> > # after route add: check route get $addr is not unreachable >> >> > while true; do >> >> > ip -6 route del $tsubnet::/64 dev dum0 >> >> > ip -6 route add $tsubnet::/64 dev dum0 >> >> > r=`ip -6 -d -o route get $taddr` >> >> > if echo "$r" | grep -q unreachable ; then >> >> > echo >> >> > echo >> >> > echo BUG: `uname -a` >> >> > echo BUG: Got unexpected unreachable route for $taddr: >> >> > echo "$r" >> >> > echo >> >> > echo "route table for root $tprefix::/48" >> >> > echo "---- 8< ----" >> >> > ip -6 -d -o route show root $tprefix::/48 >> >> > echo "---- 8< ----" >> >> > echo >> >> > echo "route for $taddr (once again)" >> >> > ip -6 -d -o -s -s -s route get $taddr >> >> > exit 1 >> >> > fi >> >> > done -- Dave Täht Let's go make home routers and wifi faster! With better software! http://blog.cerowrt.org _______________________________________________ Babel-users mailing list Babel-users@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/babel-users