Hi Tim,

The Squeak process scheduler usually preempts a process only by  
another one of higher priority. And when the scheduler is ready to go  
back at the lower priority, it will always resume the same process  
that was last suspended. The only way for two or more processes of the  
same priority to share the processor is when the running process calls  
#wait or #yield.

This means that using shared state from several processes at the same  
priority (without using any locking mechanism, such as Semaphores,  
Monitors or critical sections) is safe, as long as the processes do  
#wait or #yield only when that shared state is in a consistent state.

It looks like this ARM VM is somehow not following this convention. If  
this was the case, this kind of problems could arise. I just answered  
with the same text to a thread in VM-Dev, where the problem shows up  
when doing Morphic stuff (in Cuis) from two different processes of the  
same priority. Seeing it again in a different part of a different  
system, but on the same platform made me realize it must be a VM issue.

In any case, if you do your proposed change, please do it in a new  
method called #computeAttackSustainValueAtMSecs: or such.  
#computeValueAtMSecs: is used (at least in Cuis, jm 2/4/98) in  
#showOnDisplay, that draws the envelope including the decay phase. But  
if I'm right, until the assumption on the behavior of the scheduled is  
'fixed', more weird shared state problems will arise.

HTH,
Juan Vuletich

Quoting tim Rowledge <tim at rowledge.org>:

> To partly answer my own question, I just noticed that  
> Envelope>computeValueAtMSecs: is only sent by Envelope>sustainEnd:  
> and thus that first clause must always get skipped (because we  
> always call it with loopEndMSecs== nil) and so surely we’d be ok to  
> delete that bit and not mess around with the ‘pretend to be  
> sustaining’ stuff?
>
> tim
> --
> tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
> Useful random insult:- Calls people to ask them their phone number.