Re: sinval synchronization considered harmful

On Fri, Jul 22, 2011 at 3:28 PM, Noah Misch <noah(at)2ndquadrant(dot)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
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