On Fri, Jul 22, 2011 at 3:28 PM, Noah Misch <noah@2ndquadrant.com> wrote:
> On Thu, Jul 21, 2011 at 11:37:27PM -0400, Robert Haas wrote:
>> I think I have a simpler idea, though:
>> before acquiring any locks, just have SIGetDataEntries() do this:
>>
>> + if (stateP->nextMsgNum == segP->maxMsgNum && !stateP->resetState)
>> + return 0;
>>
>> Patch (with comment explaining why I think this is OK) attached. If
>> the message numbers happen to be equal only because the counter has
>> wrapped, then stateP->resetState will be true, so we'll still realize
>> we need to do some work.
>
> This is attractive, and I don't see any problems with it. (In theory, you could
> hit a case where the load of resetState gives an ancient "false" just as the
> counters wrap to match. Given that the wrap interval is 1000000x as long as the
> reset interval, I'm not worried about problems on actual silicon.)
It's actually 262,144 times as long - see MSGNUMWRAPAROUND.
It would be pretty easy to eliminate even the theoretical possibility
of a race by getting rid of resetState altogether and using nextMsgNum
= -1 to mean that. Maybe I should go ahead and do that.
> +1 for doing this and moving on.
Yeah, I think I'll go ahead and commit something along these lines if
no one objects. We can always fine-tune it more if needed (but it
probably isn't).
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company