Re: Intermediate report for AIX 5L port

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom, I assume what you are saying is that the access to the spinlocks,
> > already marked as volatile, should have prevented any code from
> > migrating over those locks.  I guess my big question is does any
> > volatile access prevent optimization of other variables across that
> > volatiles access?  I didn't think that was guaranteed.
> 
> After eyeballing the C spec some more, I think you might be right.
> If that's the correct reading then it is indeed necessary for lwlock.c
> to mark the whole lock structure as volatile, not only the spinlock
> fields.

OK.

> However, if that's true then (a) 7.2 has three other modules that are
> potentially vulnerable to similar problems; (b) prior releases had

That was going to be my next question.

> many more places that were potentially vulnerable --- ie, all the
> modules that used to use spinlocks directly and now use LWLocks.
> If this sort of behavior is allowed, ISTM we should have been seeing
> major instability on lots of SMP machines.

Again, a good question.  No idea.

Here is a more general question:

If you do:
get lock;a=4release lock;

Can the compiler reorder that to:
a=4get lock;release lock;

It can see the lock values don't have any effect on 'a'.  What actually
does keep this stuff from moving around?


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