Date: Thu, 02 Apr 2015 07:53:07 -0600 From: Ian Lepore <ian@freebsd.org> To: Gleb Smirnoff <glebius@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r280971 - in head: contrib/ipfilter/tools share/man/man4 sys/contrib/ipfilter/netinet sys/netinet sys/netipsec sys/netpfil/pf Message-ID: <1427982787.82583.111.camel@freebsd.org> In-Reply-To: <20150402123522.GC64665@FreeBSD.org> References: <201504012226.t31MQedN044443@svn.freebsd.org> <1427929676.82583.103.camel@freebsd.org> <20150402123522.GC64665@FreeBSD.org>
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On Thu, 2015-04-02 at 15:35 +0300, Gleb Smirnoff wrote: > On Wed, Apr 01, 2015 at 05:07:56PM -0600, Ian Lepore wrote: > I> > Author: glebius > I> > Date: Wed Apr 1 22:26:39 2015 > I> > New Revision: 280971 > I> > URL: https://svnweb.freebsd.org/changeset/base/280971 > I> > > I> > Log: > I> > o Use new function ip_fillid() in all places throughout the kernel, > I> > where we want to create a new IP datagram. > I> > o Add support for RFC6864, which allows to set IP ID for atomic IP > I> > datagrams to any value, to improve performance. The behaviour is > I> > controlled by net.inet.ip.rfc6864 sysctl knob, which is enabled by > I> > default. > I> > o In case if we generate IP ID, use counter(9) to improve performance. > I> > o Gather all code related to IP ID into ip_id.c. > I> > > I> > Differential Revision: https://reviews.freebsd.org/D2177 > I> > Reviewed by: adrian, cy, rpaulo > I> > Tested by: Emeric POUPON <emeric.poupon stormshield.eu> > I> > Sponsored by: Netflix > I> > Sponsored by: Nginx, Inc. > I> > Relnotes: yes > I> > > I> [...] > I> > +void > I> > +ip_fillid(struct ip *ip) > I> > +{ > I> > + > I> > + /* > I> > + * Per RFC6864 Section 4 > I> > + * > I> > + * o Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0) > I> > + * o Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0) > I> > + */ > I> > + if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF)) > I> > + ip->ip_id = 0; > I> > + else if (V_ip_do_randomid) > I> > + ip->ip_id = ip_randomid(); > I> > + else { > I> > + counter_u64_add(V_ip_id, 1); > I> > + ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff); > I> > + } > I> > +} > I> > + > I> > I> This is completely bogus. It's a big opacity violation (it relies on > I> what should be opaque private internal implementation details of > I> counter(9)). The fact that the counter api doesn't provide a function > I> for retrieving one cpu's counter value should be a big clue there -- the > I> fact that you know the internals doesn't make it okay to reach behind > I> the counter and grab a value like that. It may not even be safe to do > I> so on any given architecture; it certainly isn't safe on arm, and that > I> line of code above will work only by accident because you're throwing > I> way all but 16 bits. > > I though about providing that API, but since it isn't safe in general, > I decided to not do that. > > I> But even more importantly, this WILL result in multiple threads using > I> the same value at the same time... > I> > I> - Thread A on CPU 1 and thread B on CPU 2 both begin executing here at > I> the same time, and both get through counter_u64_add(). > I> - Thread A keeps running and uses CPU 1's new value, call it 27. > I> - Thread B gets prempted between counter_u64_add() and zpcpu_get(). > I> When it resumes it's now on CPU 1, so it retrieves value 27 as well. > > This was already discussed in this thread: > > https://lists.freebsd.org/pipermail/svn-src-head/2015-March/069864.html > Discussed, but not really resolved, or even reasonably addressed. But I guess if you think it's done, more words at this point aren't going to make you see the problems clearly enough to make the required changes. -- Ian
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