Re: some question about deadlock

Hi,    In your example, it seems that process B is the first such waiter( the request of B conflicts AccessShareLock).

Best regards.

----- Original Message ----- 
From: "Tom Lane" <tgl@sss.pgh.pa.us>
To: "ipig" <ipig@ercist.iscas.ac.cn>
Cc: "Bruce Momjian" <pgman@candle.pha.pa.us>; <pgsql-hackers@postgresql.org>
Sent: Monday, May 29, 2006 11:51 PM
Subject: Re: [HACKERS] some question about deadlock 


> "ipig" <ipig@ercist.iscas.ac.cn> writes:
>>     That is to say, if p0 wants to lock A again, then p0 will be put before p1, and p0 will be at the head of the
queue.Why do we need to find the first waiter which conflicts p0? I think that p0 must be added at the head of the wait
queue.
> 
> Your analysis is assuming that there are only two kinds of lock, which
> is not so.  Process A might hold a weak lock and process B a slightly
> stronger lock that doesn't conflict with A's.  In the wait queue there
> might be process C wanting a lock that conflicts with B's but not A's,
> followed by process D wanting a strong lock that conflicts with all three.
> Now suppose A wants to get a lock of the same type D wants.  Since this
> conflicts with B's existing lock, A must wait.  A must go into the queue
> before D (else deadlock) but if possible it should go after C, on
> fairness grounds.
> 
> A concrete example here is
> A has AccessShareLock (reader's lock)
> B has RowExclusiveLock (writer's lock)
> C wants ShareLock (hence blocked by B but not A)
> D wants AccessExclusiveLock (must wait for all three)
> If A wants to upgrade to AccessExclusiveLock, it *must* queue in front
> of D, and we'd prefer that it queue behind C not in front of C.
> 
> regards, tom lane
> 
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