Re: mutexes, locks and so on...

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

On Fri, Nov 12, 2010 at 02:58:16PM +0100, Johnny Billquist wrote:
> >lock(interlock)
> >lock(a)
> >lock(b)
> >unlock(interlock)
> 
> I realized I'm getting sloppy here. When I say locks in this
> context, I'm actually talking about spin mutexes. Simple locks are a
> different story, since they don't fool with IPL levels at all.
> 
> Also, for spin mutexes I'm not sure the above scenario would make
> sense. Since, if they were spin mutexes, assuming once process did
> that, and then another did it, the second process would stop at
> lock(a), with interlock held. I would assume interlock would be at a
> very high ipl, so the first process would never continue, and unlock
> a and b, so we'd be in a deadlock.
> And if the idea is that we'd stay at the high ipl in the first
> process, that more or less would mean that we essentially holds on
> to interlock, so why unlock it?

If you have a UP system, it doesn't make a difference, yes. For SMP, it
does. It allows other CPUs to continue if other interface, even using
the interlock.

Joerg