On Fri, May 19, 2017 at 2:05 PM, Michael Paquier <michael.paqu...@gmail.com> wrote: > On Thu, May 18, 2017 at 1:43 PM, Thomas Munro > <thomas.mu...@enterprisedb.com> wrote: >> On Thu, May 11, 2017 at 1:48 PM, Michael Paquier >> <michael.paqu...@gmail.com> wrote: >>> I had my eyes on the WAL sender code this morning, and I have noticed >>> that walsender.c is not completely consistent with the PID lookups it >>> does in walsender.c. In two code paths, the PID value is checked >>> without holding the WAL sender spin lock (WalSndRqstFileReload and >>> pg_stat_get_wal_senders), which looks like a very bad idea contrary to >>> what the new WalSndWaitStopping() does and what InitWalSenderSlot() is >>> doing for ages. >> >> There is also code that accesses shared walsender state without >> spinlocks over in syncrep.c. I think that file could use a few words >> of explanation for why it's OK to access pid, state and flush without >> synchronisation. > > Yes, that is read during the quorum and priority sync evaluation. > Except sync_standby_priority, all the other variables should be > protected using the spin lock of the WAL sender. walsender_private.h > is clear regarding that. So the current coding is inconsistent even > there. Attached is an updated patch.
I don't claim that that stuff is wrong, just that it would be good to hear an analysis. I think the question is: is there a way for syncrep to hang because of a perfectly timed walsender transition, however vanishingly unlikely? I'm thinking of something like: syncrep skips a walsender slot because it's looking at arbitrarily old 'pid' from before a walsender connected, or gets a torn read of 'flush' that comes out as 0 but was actually non-0. Incidentally, I suspect that a couple of places where 'volatile' is used it's superfluous (accessing things protected by an LWLock that is held). I don't see any of this as 'open item' material, it's interesting to look into but it's preexisting code. As for unlocked reads used for pg_stat_X views, it seems well established that we're OK with that (at least for things that the project has decided can be read atomically, to wit aligned 32 bit values). -- Thomas Munro http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
