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

On Mon, 30 Jul 2001, Hiroshi Inoue wrote:

> I have a question.
> What will happen when the second table is locked for a long time
> though the first table isn't locked ?

Consider the case:

LOCK a,b;

Assume a is free (i.e. not locked), but b is busy (i.e. locked).

First the system will do a blocking lock attempt on a, which will return
immediately, since a was free. Table a is now locked. Now, the system will
try a non-blocking lock on b. But, b is busy so the lock attempt will fail
immediately (since the lock attempt was non-blocking). So, the system will
back off, and the lock on a is released.

Next, a blocking lock attempt will be made on b. (Since it was busy last
time, we want to wait for it to become free.) The lock call will block
until b becomes free. At that time, the lock attempt will return, and b
will be locked. Then, a non-blocking lock attempt will be made on table a.
(Recall that we don't have a lock on it, since we released it during
back-off earlier.) Assuming a is still free, it will be locked and the
LOCK command will complete. Otherwise, if a is busy, the lock attempt will
then restart with a blocking lock attempt on a. The procedure will
continue until all tables are free to lock.

In summary, no locks are held while waiting for tables to become free --
in essence, the tables are locked all at once, once all tables in the
LOCK statement are free.

Neil

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Neil Padgett
Red Hat Canada Ltd. E-Mail: npadgett(at)redhat(dot)com
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