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! > >> 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 -- 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