[ovs-dev] [PATCH] rconn: Introduce new invariant to fix assertion failure in corner case.

Ben Pfaff blp at ovn.org
Mon Jun 18 15:07:21 UTC 2018


On Mon, Jun 18, 2018 at 05:18:49PM +0300, Ilya Maximets wrote:
> > On Wed, May 23, 2018 at 09:28:59PM -0700, Ben Pfaff wrote:
> >> On Wed, May 23, 2018 at 06:06:44PM -0700, Han Zhou wrote:
> >> > On Wed, May 23, 2018 at 5:14 PM, Ben Pfaff <blp at ovn.org> wrote:
> >> > >
> >> > > Until now, rconn_get_version() has only reported the OpenFlow version in
> >> > > use when the rconn is actually connected.  This makes sense, but it has a
> >> > > harsh consequence.  Consider code like this:
> >> > >
> >> > >     if (rconn_is_connected(rconn) && rconn_get_version(rconn) >= 0) {
> >> > >         for (int i = 0; i < 2; i++) {
> >> > >             struct ofpbuf *b = ofputil_encode_echo_request(
> >> > >                 rconn_get_version(rconn));
> >> > >             rconn_send(rconn, b, NULL);
> >> > >         }
> >> > >     }
> >> > >
> >> > > Maybe not the smartest code in the world, and probably no one would write
> >> > > this exact code in any case, but it doesn't look too risky or crazy.
> >> > >
> >> > > But it is.  The second trip through the loop can assert-fail inside
> >> > > ofputil_encode_echo_request() because rconn_get_version(rconn) returns -1
> >> > > instead of a valid OpenFlow version.  That happens if the first call to
> >> > > rconn_send() encounters an error while sending the message and therefore
> >> > > destroys the underlying vconn and disconnects so that rconn_get_version()
> >> > > doesn't have a vconn to query for its version.
> >> > >
> >> > > In a case like this where all the code to send the messages is close by,
> >> > we
> >> > > could just check rconn_get_version() in each loop iteration.  We could
> >> > even
> >> > > go through the tree and convince ourselves that individual bits of code
> >> > are
> >> > > safe, or be conservative and check rconn_get_version() >= 0 in the iffy
> >> > > cases.  But this seems to me like an ongoing source of risk and a way to
> >> > > get things wrong in corner cases.
> >> > >
> >> > > This commit takes a different approach.  It introduces a new invariant: if
> >> > > an rconn has ever been connected, then it returns a valid OpenFlow version
> >> > > from rconn_get_version().  In addition, if an rconn is currently
> >> > connected,
> >> > > then the OpenFlow version it returns is the correct one (that may be
> >> > > obvious, but there were corner cases before where it returned -1 even
> >> > > though rconn_is_connected() returned true).
> >> > >
> >> > > With this commit, the code above would work OK.  If the first call to
> >> > > rconn_send() encounters an error sending the message, then
> >> > > rconn_get_version() in the second iteration will return the same value as
> >> > > in the first iteration.  The message passed to rconn_send() will end up
> >> > > being discarded, but that's much better than either an assertion failure
> >> > or
> >> > > having to carefully analyze a lot of our code to deal with one unusual
> >> > > corner case.
> >> > >
> >> > > Reported-by: Han Zhou <zhouhan at gmail.com>
> >> > > Signed-off-by: Ben Pfaff <blp at ovn.org>
> >> > > ---
> >> > >  lib/learning-switch.c |  2 +-
> >> > >  lib/rconn.c           | 41 ++++++++++++++++-------------------------
> >> > >  lib/vconn.c           |  1 +
> >> > >  3 files changed, 18 insertions(+), 26 deletions(-)
> >> > >
> >> > Acked-by: Han Zhou <hzhou8 at ebay.com>
> >> 
> >> Thanks.  I applied this to master.  I'll backport it to older versions
> >> if no one notices trouble soon.
> > 
> > I backported as far as branch-2.5.
> 
> Sorry for being late, but I just tried to use branch-2.9 on my test environment
> and found that this patch produces a lot of "connected" logs that are a bit annoying:
> 
> 2018-06-18T12:55:06.610Z|03051|rconn|INFO|br-int<->unix#2873: connected
> 2018-06-18T12:55:08.609Z|03052|rconn|INFO|br-int<->unix#2874: connected
> 2018-06-18T12:55:08.610Z|03053|rconn|INFO|br-int<->unix#2875: connected
> 2018-06-18T12:55:10.608Z|03054|rconn|INFO|br-int<->unix#2876: connected
> 2018-06-18T12:55:10.609Z|03055|rconn|INFO|br-int<->unix#2877: connected
> 2018-06-18T12:55:12.609Z|03056|rconn|INFO|br-int<->unix#2878: connected
> 2018-06-18T12:55:12.609Z|03057|rconn|INFO|br-int<->unix#2879: connected
> 2018-06-18T12:55:14.612Z|03058|rconn|INFO|br-int<->unix#2880: connected
> 2018-06-18T12:55:14.613Z|03059|rconn|INFO|br-int<->unix#2881: connected
> 2018-06-18T12:55:16.613Z|03060|rconn|INFO|br-int<->unix#2882: connected
> 2018-06-18T12:55:16.614Z|03061|rconn|INFO|br-int<->unix#2883: connected
> 2018-06-18T12:55:18.609Z|03062|rconn|INFO|br-int<->unix#2884: connected
> 2018-06-18T12:55:18.610Z|03063|rconn|INFO|br-int<->unix#2885: connected
> 2018-06-18T12:55:20.610Z|03064|rconn|INFO|br-int<->unix#2886: connected
> 2018-06-18T12:55:20.611Z|03065|rconn|INFO|br-int<->unix#2887: connected
> 
> On my system it's about 2 log messages on each 2 seconds. It has only
> ovsdb-server, ovs-vswitchd and ovsdb-client monitor running.
> 
> Could you please check?

Are you sure that it's this patch that makes a difference?  It doesn't
change anything in the logging.


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