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

Ilya Maximets i.maximets at samsung.com
Tue Jun 19 07:53:00 UTC 2018


On 18.06.2018 22:25, Ben Pfaff wrote:
> On Mon, Jun 18, 2018 at 09:31:43AM -0700, Han Zhou wrote:
>> On Mon, Jun 18, 2018 at 8:34 AM, Ilya Maximets <i.maximets at samsung.com>
>> 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".
>>>
>> Hi Ilya,
>>
>> From the debug log it looks like the connection is flapping, even without
>> the patch. The patch just revealed the problem by an info level log.
> 
> Probably these are just connections from "ovs-ofctl" or similar.  We try
> to suppress logging for them because it isn't too useful.

Yes, you're right. There was some monitoring tool running that uses ovs-ofctl.
I just didn't recognize it at the first glance.

Best regards, Ilya Maximets.


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