Re: Some bogus results from prairiedog

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jul 22, 2014 at 12:24 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Anyway, to cut to the chase, the crash seems to be from this:
>> TRAP: FailedAssertion("!(FastPathStrongRelationLocks->count[fasthashcode] > 0)", File: "lock.c", Line: 2957)
>> So there is still something rotten in the fastpath lock logic.

> Gosh, that sucks.

> The inconstancy of this problem would seem to suggest some kind of
> locking bug rather than a flat-out concurrency issue, but it looks to
> me like everything relevant is marked volatile.

I don't think that you need any big assumptions about machine-specific
coding issues to spot the problem. The assert in question is here:

/*
* Decrement strong lock count. This logic is needed only for 2PC.
*/
if (decrement_strong_lock_count
&& ConflictsWithRelationFastPath(&lock->tag, lockmode))
{
uint32 fasthashcode = FastPathStrongLockHashPartition(hashcode);

SpinLockAcquire(&FastPathStrongRelationLocks->mutex);
Assert(FastPathStrongRelationLocks->count[fasthashcode] > 0);
FastPathStrongRelationLocks->count[fasthashcode]--;
SpinLockRelease(&FastPathStrongRelationLocks->mutex);
}

and it sure looks to me like that
"ConflictsWithRelationFastPath(&lock->tag" is looking at the tag of the
shared-memory lock object you just released. If someone else had managed
to recycle that locktable entry for some other purpose, the
ConflictsWithRelationFastPath call might incorrectly return true.

I think s/&lock->tag/locktag/ would fix it, but maybe I'm missing
something.

regards, tom lane