Re: BUG #13985: Segmentation fault on PREPARE TRANSACTION

Andres Freund <andres@anarazel.de> writes:
> On 2016-02-25 09:51:49 -0500, Tom Lane wrote:
>> Marking pgprocno volatile is silly.  What *is* missing is this:
>>
>> -    ProcArrayStruct *arrayP = procArray;
>> +    volatile ProcArrayStruct *arrayP = procArray;

> Well, that'll also force arrayP->numProcs to be loaded from memory every
> loop iteration. Not sure if we really want that.

I think we do.  The entire point here is not to assume that that storage
isn't changing.

> What bothers me about this right now is that we currently write the
> pgprocno array with:
>     memmove(&arrayP->pgprocnos[index + 1], &arrayP->pgprocnos[index],
>             (arrayP->numProcs - index) * sizeof(int));
>     arrayP->pgprocnos[index] = proc->pgprocno;
>     arrayP->numProcs++;
> afaics there's absolutely no guarantee that memmov() will only use
> aligned sizeof(int) writes.

Ugh.  That's a separate problem, but yes, it's a problem.

Seems like we can either (1) get rid of that memmove in favor of
a handwritten loop, or (2) give up on unlocked access to the
pgprocnos array.  Which performance hit would you rather take?

            regards, tom lane