On Sat, Dec 31, 2016 at 12:44 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: > During the review of Group update Clog patch , Dilip noticed an > issue with the patch where it can leak the semaphore count in one of > the corner case. I have checked and found that similar issue exists > for Group clear xid (ProcArrayGroupClearXid) as well. I think the > reason why this problem is not noticed by anyone till now is that it > can happen only in a rare scenario when the backend waiting for xid > clear is woken up due to some unrelated signal. This problem didn't > exist in the original commit > (0e141c0fbb211bdd23783afa731e3eef95c9ad7a) of the patch, but later > while fixing some issues in the committed patch, it got introduced in > commit 4aec49899e5782247e134f94ce1c6ee926f88e1c. Patch to fix the > issue is attached.
Yeah, that looks like a bug. Thanks for the detailed analysis; committed and back-patched to 9.6. I suppose in the worst case it's possible that we'd leak a semaphore count and then every future time we enter a PGSemaphoreLock using that PGPROC we have to eat up the leaked count (or counts) and then put it (or them) back after we really wait. That would suck. But I wasn't able to observe any leaks in a high-concurrency pgbench test on hydra, so it's either very unlikely or requires some additional condition to trigger the problem. I think we have run into this kind of issue before. I wonder if there's any way to insert some kind of a guard - e.g. detect at backend startup time that the semaphore has a non-zero value and fix it, issuing a warning along the way... maybe something like: while (sem_trywait(sem) == 0) ++bogus; if (bogus > 0) elog(WARNING, "uh oh"); I'm not sure if that would be prone to false positives though. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers