Re: Soft deadlocks

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> I'm trying to understand what a soft deadlock is as described by deadlock.c.

> As best I understand if a process, A, is waiting for a lock and is being
> blocked only because someone, B, is ahead of it in the queue but hasn't been
> granted the conflicting lock we want to jump A ahead of B.

No, we only do that if it breaks a deadlock. In your example there is
no reason to move process A ahead of B.

A more typical situation is like this:

Process A:
begin;
select * from T where ...;
-- now A holds AccessShareLock on T

Process B:
lock table T;
-- wants AccessExclusiveLock on T, blocks waiting for A

Process A:
lock table T;
-- blocks behind B?

Fairness would normally demand that A queue behind B for the
AccessExclusiveLock, but if we do that we have a deadlock.
So we spring A ahead of B and let it have the AccessExclusiveLock
out of turn.

This is just the base case; you can get into similar situations
involving more than one lockable object and more than two processes.
I believe that the above case is caught by the test in ProcSleep and
A will be granted the lock upgrade without blocking at all; but any
more-complex situation will only be discovered when someone runs the
deadlock checker.

regards, tom lane