Re: some question about deadlock

ipig wrote:
> Hi,
>
> Below is the notes from postgresql-8.1.3/src/backend/storage/lmgr/README:
>
>
> Lock acquisition (routines LockAcquire and ProcSleep) follows these rules:
>
> 1. A lock request is granted immediately if it does not conflict with
> any existing or waiting lock request, or if the process already holds an
> instance of the same lock type (eg, there's no penalty to acquire a read
> lock twice). Note that a process never conflicts with itself, eg one
> can obtain read lock when one already holds exclusive lock.
>
> 2. Otherwise the process joins the lock's wait queue. Normally it will
> be added to the end of the queue, but there is an exception: if the
> process already holds locks on this same lockable object that conflict
> with the request of any pending waiter, then the process will be
> inserted in the wait queue just ahead of the first such waiter. (If we
> did not make this check, the deadlock detection code would adjust the
> queue order to resolve the conflict, but it's relatively cheap to make
> the check in ProcSleep and avoid a deadlock timeout delay in this case.)
> Note special case when inserting before the end of the queue: if the
> process's request does not conflict with any existing lock nor any
> waiting request before its insertion point, then go ahead and grant the
> lock without waiting.
>
>
> I am confused with that exception(in bold), could some one give me an example?

Well, first you are requiring people to be reading HTML email to see the
bold text you added, which is not good:

an exception: if the process already holds locks on this same lockable
object that conflict with the request of any pending waiter, then the
process will be inserted in the wait queue just ahead of the first such
waiter.

An example would be you have a read lock and want an exclusive lock, and
someone else in the queue is waiting for a read lock too. In this case,
the exclusive lock goes before the queued read lock.

--
Bruce Momjian http://candle.pha.pa.us
EnterpriseDB http://www.enterprisedb.com

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