Обсуждение: Patch for LISTEN/NOTIFY race condition?
Hi all, After upgrading to 8.3 and Slony 1.2.13, one of our import processes started constantly failing because of deadlock. Our import process (pl/pgsql function) just uses COPY FROM to populate a temp table and then inserts from that (without any special locks, etc). And, the deadlock error showed that the import process was waiting on sl_event and a slon process was waiting on pg_listener. I found the following thread in the archives: http://archives.postgresql.org/pgsql-bugs/2008-03/msg00117.php and it seemed like it could be the cause of the problem. Is the patch available anywhere? Also, if it will be available in 8.3.2, is there any information on when that will be released? Thanks and sorry if I missed this information posted somewhere, Meetesh
Meetesh Karia wrote: > Hi all, > > After upgrading to 8.3 and Slony 1.2.13, one of our import processes > started constantly failing because of deadlock. Our import process > (pl/pgsql function) just uses COPY FROM to populate a temp table and > then inserts from that (without any special locks, etc). And, the > deadlock error showed that the import process was waiting on sl_event > and a slon process was waiting on pg_listener. > > I found the following thread in the archives: > http://archives.postgresql.org/pgsql-bugs/2008-03/msg00117.php and it > seemed like it could be the cause of the problem. > > Is the patch available anywhere? Also, if it will be available in > 8.3.2, is there any information on when that will be released? Hmm, AFAICS the patch should be present on 8.3.1, because it was tagged on March 14th and the patch was committed on March 12. 2008-03-12 17:11 tgl * doc/src/sgml/ref/prepare_transaction.sgml (1.6.2.1), src/backend/commands/async.c (1.138.2.1): Fix LISTEN/NOTIFY race condition reported by Laurent Birtz, by postponing pg_listener modifications commanded by LISTEN and UNLISTEN until the end of the current transaction. This allows us to hold the ExclusiveLock on pg_listener until after commit, with no greater risk of deadlock than there was before. Aside from fixing the race condition, this gets rid of a truly ugly kludge that was there before, namely having to ignore HeapTupleBeingUpdated failures during NOTIFY. There is a small potential incompatibility, which is that if a transaction issues LISTEN or UNLISTEN and then looks into pg_listener before committing, it won't see any resulting row insertion or deletion, where before it would have. It seems unlikely that anyone would be depending on that, though. This patch also disallows LISTEN and UNLISTEN inside a prepared transaction. That case had some pretty undesirable properties already, such as possibly allowing pg_listener entries to be made for PIDs no longer present, so disallowing it seems like a better idea than trying to maintain the behavior. -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support
Alvaro Herrera <alvherre@commandprompt.com> writes: > Meetesh Karia wrote: >> After upgrading to 8.3 and Slony 1.2.13, one of our import processes >> started constantly failing because of deadlock. Our import process >> (pl/pgsql function) just uses COPY FROM to populate a temp table and >> then inserts from that (without any special locks, etc). And, the >> deadlock error showed that the import process was waiting on sl_event >> and a slon process was waiting on pg_listener. >> >> I found the following thread in the archives: >> http://archives.postgresql.org/pgsql-bugs/2008-03/msg00117.php and it >> seemed like it could be the cause of the problem. >> >> Is the patch available anywhere? Also, if it will be available in >> 8.3.2, is there any information on when that will be released? > Hmm, AFAICS the patch should be present on 8.3.1, because it was tagged > on March 14th and the patch was committed on March 12. Yes, but in any case (a) that's about a race condition not a deadlock, and (b) the issue has been there since forever, so it is unlikely to explain or fix a problem that you weren't seeing before 8.3. regards, tom lane
Thanks all ... I'm going to do some more digging and see if I can isolate the problem. Meetesh Tom Lane wrote: > Alvaro Herrera <alvherre@commandprompt.com> writes: > >> Meetesh Karia wrote: >> >>> After upgrading to 8.3 and Slony 1.2.13, one of our import processes >>> started constantly failing because of deadlock. Our import process >>> (pl/pgsql function) just uses COPY FROM to populate a temp table and >>> then inserts from that (without any special locks, etc). And, the >>> deadlock error showed that the import process was waiting on sl_event >>> and a slon process was waiting on pg_listener. >>> >>> I found the following thread in the archives: >>> http://archives.postgresql.org/pgsql-bugs/2008-03/msg00117.php and it >>> seemed like it could be the cause of the problem. >>> >>> Is the patch available anywhere? Also, if it will be available in >>> 8.3.2, is there any information on when that will be released? >>> > > >> Hmm, AFAICS the patch should be present on 8.3.1, because it was tagged >> on March 14th and the patch was committed on March 12. >> > > Yes, but in any case (a) that's about a race condition not a deadlock, > and (b) the issue has been there since forever, so it is unlikely to > explain or fix a problem that you weren't seeing before 8.3. > > regards, tom lane >