Re: Revised Patch to allow multiple table locks in "Unison"

Your patch has been added to the PostgreSQL unapplied patches list at:

    http://candle.pha.pa.us/cgi-bin/pgpatches

I will try to apply it within the next 48 hours.

> Bruce Momjian wrote:
> >
> > The problem with this patch is that it doesn't always lock the tables in
> > the order supplied by the user, leading to possible deadlock.  My guess
> > is that you need to try locking A, B, C and if B hangs, I think you need
> > to sleep on B, and when you get it, release the lock on B and try A, B,
> > C again.  I know this is a pain and could fail multiple times, but I
> > think we have to do it this ay.
> >
>
> Deadlocks are not possible with this patch. The four conditions needed
> for deadlock are (according to Operating Systems: Internals and Design
> Principles, 4th Ed. by W. Stallings):
>
> 1. Mutual exclusion: Only one process may use a resource at a time.
> 2. Hold and wait: A process may hold allocated resources while awaiting
> assignment of others.
> 3. No preemption: No resources can be forcibly removed from a process
> holding it.
> 4. Circular wait:  A closed chain of processes exists, such that each
> process holds at least one resource needed by the next process in the
> chain.
>
> For deadlock prevention one needs only to prevent the existence of
> at least one of the four conditions.
>
>
> The patch code never holds any of requested locks, while waiting for a
> locked relation to become free. If a lock on all tables in the lock list
> cannot be acquired at once, it backs off and releases all locks.
>
> Stallings writes about preventing condition 3: "This condition can be
> prevented in several ways. [. . .] [One way is to require that,] if a
> process holding certain resources is denied a further request, that
> process must release its original resources and, if necessary, request
> them again together with the additional resources."
>
> This is exactly what the patch does. Observe that if one lock is not
> available, the patch releases all locks so far acquired, and then
> acquires
> the locks again. Hence, condition 3 is prevented, and so deadlock is
> prevented.
>
> Neil
>
> p.s. Is this mailing list always this slow?
>
> --
> Neil Padgett
> Red Hat Canada Ltd.                       E-Mail:  npadgett@redhat.com
> 2323 Yonge Street, Suite #300,
> Toronto, ON  M4P 2C9
>

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