> -----Original Message-----
> From: Bruce Momjian [mailto:maillist@candle.pha.pa.us]
> Sent: Tuesday, April 27, 1999 1:33 PM
> To: Hiroshi Inoue
> Cc: pgsql-hackers@postgreSQL.org
> Subject: Re: [HACKERS] Lock freeze ? in MVCC
>
>
> OK, let me comment on this. It does not to see this as a deadlock
> because session 3 really doesn't have a lock at the point it is hanging.
> A deadlock would be if 1 has a lock that 3 is waiting for, and 3 has a
> lock 1 is waiting for.
>
> Hold on, I think I see what you are saying now. It seems the locking
> code assume table-level locking, while the new code now has MVCC. I
> better look at this. This could be ugly to fix. I look for matching
I think it's a problem of table-level locking(MVCC has 8 levels of table-
locking and even select operations acquire AccessShareLock's.)
Moreover I found it's not the problem of MVCC only.
In fact I found the following case in 6.4.2.
session-1 => create table tt (id int4);
session-1 => begin;
session-1 => select * from tt;
session-2 => begin;
session-2 => select * from tt;
session-3 => begin;
session-3 => lock table tt; (blocked)
session-1 => select * from tt; (blocked)
session-2 => end;
session-2 returns immediately,but session-3 and session-1
are still blocked
Now I'm suspicious about the following code in LockResolveConflicts().
/* * We can control runtime this option. Default is lockReadPriority=0 */ if
(!lockReadPriority) { /* ------------------------ * If someone with a greater
priorityis waiting for the
lock, * do not continue and share the lock, even if we can. bjm *
------------------------ */ int myprio =
LockMethodTable[lockmethod]->ct
l->prio[lockmode]; PROC_QUEUE *waitQueue = &(lock->waitProcs); PROC *topproc = (PROC
*)
MAKE_PTR(waitQueue->links.prev);
if (waitQueue->size && topproc->prio > myprio) {
XID_PRINT("LockResolveConflicts:higher priority
proc wa
iting", result); return STATUS_FOUND; }
}
After I removed above code on trial,select operations in my example case
are not blocked.
Comments ?
Thanks.
Hiroshi Inoue
Inoue@tpf.co.jp
