Re: [AD] Allegro on multi-processor machines

[Peter Wang <tjaden@xxxxxxxxxx>]

Evert Glebbeek wrote:

> > That doesn't really solve the problem of course, but it may give a
> > hint as to what is going wrong exactly.
> >    
>
> I think the problem is that Allegro is not using proper mutex locks. What I
> don't understand is why this problem doesn't seem to show up on single processor
> machines.
>  

AFAIK, without proper thread synchronisation, two caches may have 
different ideas about the contents of the same memory address, e.g. one 
thread updates a global variable X, but the other thread still sees the 
old value for X.  I read that Allegro's mouse would "stall" on 
multiprocessor machines (before the program crashed, that is :-).  This 
could be one explanation.  I just saw that mouse_x,y are marked 
volatile, but not the internal variables _mouse_x,y.

Also, on a single processor machine threads never *really* run in 
parallel.  I think Allegro can get away with a lot more dodgy practices 
then.

> Also, can someone please explain again why with the following definitions
>  
[snip]

> XLOCK() and XUNLOCK() don't block indefinatebly?
>  

When a signal handler is called, the process is suspended for its 
duration.  When the signal handler returns, the process is resumed 
wherever it was interrupted.  So blocking the process indefinitely 
doesn't make sense.

Peter