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

Ilya Maximets i.maximets at samsung.com
Mon Jun 18 15:34:13 UTC 2018


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.


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