Re: Lots of stuck queries after upgrade to 9.4

Heikki,

On 2015-07-20 13:27:12 +0200, Andres Freund wrote:
> On 2015-07-20 13:22:42 +0200, Andres Freund wrote:
> > Hm. The problem seems to be the WaitXLogInsertionsToFinish() call in
> > XLogFlush().
>
> These are the relevant stack traces:
> db9lock/debuglog-commit.txt
> #2  0x00007f7405bd44f4 in LWLockWaitForVar (l=0x7f70f2ab6680, valptr=0x7f70f2ab66a0, oldval=<optimized out>,
newval=0xffffffffffffffff)at /tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/storage/lmgr/lwlock.c:1011 
> #3  0x00007f7405a0d3e6 in WaitXLogInsertionsToFinish (upto=121713318915952) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/access/transam/xlog.c:1755
> #4  0x00007f7405a0e1d3 in XLogFlush (record=121713318911056) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/access/transam/xlog.c:2849
>
> db9lock/debuglog-insert-8276.txt
> #1  0x00007f7405b77d91 in PGSemaphoreLock (sema=0x7f73ff6531d0, interruptOK=0 '\000') at pg_sema.c:421
> #2  0x00007f7405bd4849 in LWLockAcquireCommon (val=<optimized out>, valptr=<optimized out>, mode=<optimized out>,
l=<optimizedout>) at /tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/storage/lmgr/lwlock.c:626 
> #3  LWLockAcquire (l=0x7f70ecaaa1a0, mode=LW_EXCLUSIVE) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/storage/lmgr/lwlock.c:467
> #4  0x00007f7405a0dcca in AdvanceXLInsertBuffer (upto=<optimized out>, opportunistic=<optimized out>) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/access/transam/xlog.c:2161
> #5  0x00007f7405a0e301 in GetXLogBuffer (ptr=121713318928384) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/access/transam/xlog.c:1848
> #6  0x00007f7405a0e9c9 in CopyXLogRecordToWAL (EndPos=<optimized out>, StartPos=<optimized out>,
rdata=0x7ffff1c21b90,isLogSwitch=<optimized out>, write_len=<optimized out>) at
/tmp/buildd/postgresql-9.4-9.4.4/build/../src/backend/access/transam/xlog.c:1494
> #7  XLogInsert (rmid=<optimized out>, info=<optimized out>, rdata=<optimized out>) at /tmp/buildd/postgre


XLogFlush() has the following comment:
            /*
             * Re-check how far we can now flush the WAL. It's generally not
             * safe to call WaitXLogInsertionsToFinish while holding
             * WALWriteLock, because an in-progress insertion might need to
             * also grab WALWriteLock to make progress. But we know that all
             * the insertions up to insertpos have already finished, because
             * that's what the earlier WaitXLogInsertionsToFinish() returned.
             * We're only calling it again to allow insertpos to be moved
             * further forward, not to actually wait for anyone.
             */
            insertpos = WaitXLogInsertionsToFinish(insertpos);

but I don't think that's valid reasoning. WaitXLogInsertionsToFinish()
calls LWLockWaitForVar(oldval = InvalidXLogRecPtr), which will block if
there's a exlusive locker and some backend doesn't yet have set
initializedUpto. Which seems like a ossible state?