Alvaro Herrera wrote:
> ipig wrote:
> > Hi,
> > Thanks for your reply.
> > I changed the format to plain text.
> >
> > For the question, suppose that process p0 held the lock of object A, and the wait queue for A is p1,p2,p3,....,
thatprocess p1 is the first waiter in the queue.
> > Since p1 is in the wait queue, the lock p1 requests must be conflict with the lock p0 held.
> > 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.
> >
> > For your example, p0 has a read lock and wants an exclusive lock.
> > Since p0 has a read lock, then in the queue, p1 must wait an exclusive lock.
> > Then p0 will be put before p1, and p0 will be at the head of the queue.
> >
> > Is there anything I misunderstood?
>
> You missed this:
>
> "Note that a process never conflicts with itself, eg one can obtain read
> lock when one already holds exclusive lock."
>
> If p0 is holding a read lock and wants an exclusive lock, it will be
> granted right away. It will not be put in the waiting queue.
Uh, unless other processes also hold a read lock on the object. In that
case, p0 has to wait, and I think the description is saying p0 will be
put ahead of other readers waiting for the object.
-- Bruce Momjian http://candle.pha.pa.us EnterpriseDB http://www.enterprisedb.com
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