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

Ben Pfaff blp at ovn.org
Mon Jun 18 19:24:25 UTC 2018


On Mon, Jun 18, 2018 at 06:34:13PM +0300, Ilya Maximets wrote:
> On 18.06.2018 18:07, Ben Pfaff wrote:
> > 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.
> 
> I don't know much about this part of OVS. That is wat I have with reverted patch
> and debug enabled:
> 
> 2018-06-18T15:31:09.162Z|00423|rconn|DBG|br-int<->unix#150: entering ACTIVE
> 2018-06-18T15:31:09.163Z|00424|rconn|DBG|br-int<->unix#150: connection closed by peer
> 2018-06-18T15:31:09.163Z|00425|rconn|DBG|br-int<->unix#150: entering DISCONNECTED
> 2018-06-18T15:31:11.162Z|00426|rconn|DBG|br-int<->unix#151: entering ACTIVE
> 2018-06-18T15:31:11.162Z|00427|rconn|DBG|br-int<->unix#151: connection closed by peer
> 2018-06-18T15:31:11.162Z|00428|rconn|DBG|br-int<->unix#151: entering DISCONNECTED
> 2018-06-18T15:31:11.162Z|00429|rconn|DBG|br-int<->unix#152: entering ACTIVE
> 2018-06-18T15:31:11.163Z|00430|rconn|DBG|br-int<->unix#152: connection closed by peer
> 2018-06-18T15:31:11.163Z|00431|rconn|DBG|br-int<->unix#152: entering DISCONNECTED
> 
> Without revert (just branch-2.9 with patch applied):
> 
> 2018-06-18T15:26:32.366Z|00658|rconn|DBG|br-int<->unix#198: entering ACTIVE
> 2018-06-18T15:26:32.366Z|00659|rconn|DBG|br-int<->unix#198: connection closed by peer
> 2018-06-18T15:26:32.366Z|00660|rconn|DBG|br-int<->unix#198: entering DISCONNECTED
> 2018-06-18T15:26:34.365Z|00661|rconn|DBG|br-int<->unix#199: entering CONNECTING
> 2018-06-18T15:26:34.365Z|00662|rconn|INFO|br-int<->unix#199: connected
> 2018-06-18T15:26:34.365Z|00663|rconn|DBG|br-int<->unix#199: entering ACTIVE
> 2018-06-18T15:26:34.365Z|00664|rconn|DBG|br-int<->unix#199: connection closed by peer
> 2018-06-18T15:26:34.365Z|00665|rconn|DBG|br-int<->unix#199: entering DISCONNECTED
> 2018-06-18T15:26:34.365Z|00666|rconn|DBG|br-int<->unix#200: entering CONNECTING
> 2018-06-18T15:26:34.365Z|00667|rconn|INFO|br-int<->unix#200: connected
> 2018-06-18T15:26:34.365Z|00668|rconn|DBG|br-int<->unix#200: entering ACTIVE
> 2018-06-18T15:26:34.366Z|00669|rconn|DBG|br-int<->unix#200: connection closed by peer
> 2018-06-18T15:26:34.366Z|00670|rconn|DBG|br-int<->unix#200: entering DISCONNECTED
> 2018-06-18T15:26:36.365Z|00671|rconn|DBG|br-int<->unix#201: entering CONNECTING
> 2018-06-18T15:26:36.365Z|00672|rconn|INFO|br-int<->unix#201: connected
> 2018-06-18T15:26:36.365Z|00673|rconn|DBG|br-int<->unix#201: entering ACTIVE
> 2018-06-18T15:26:36.365Z|00674|rconn|DBG|br-int<->unix#201: connection closed by peer
> 2018-06-18T15:26:36.365Z|00675|rconn|DBG|br-int<->unix#201: entering DISCONNECTED
> 2018-06-18T15:26:36.365Z|00676|rconn|DBG|br-int<->unix#202: entering CONNECTING
> 2018-06-18T15:26:36.366Z|00677|rconn|INFO|br-int<->unix#202: connected
> 
> Looks like that patch just changed a workflow a bit introducing "CONNECTING"
> state in case where was just direct transition to "ACTIVE".
> 
> Best regards, Ilya Maximets.

Thanks, I understand now.

Does the following patch restore the previous behavior?

diff --git a/lib/rconn.c b/lib/rconn.c
index a3f811fedbe3..b30909fd1332 100644
--- a/lib/rconn.c
+++ b/lib/rconn.c
@@ -511,7 +511,9 @@ run_CONNECTING(struct rconn *rc)
 {
     int retval = vconn_connect(rc->vconn);
     if (!retval) {
-        VLOG_INFO("%s: connected", rc->name);
+        if (rc->reliable) {
+            VLOG_INFO("%s: connected", rc->name);
+        }
         rc->n_successful_connections++;
         state_transition(rc, S_ACTIVE);
         rc->version = vconn_get_version(rc->vconn);


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