I believe your code is incorrect.  Does Squeak use a termination 
exception to safely kill processes?  If not, ignore the ramblings of a 
lunatic.  If so, a window exists between the #wait and the #ensure: 
(right between the two statements).  If the process is terminated in 
that window, the semaphore is not released.  I have patched this in 
VisualWorks (same problem), but the makes use of some interesting 
conjectures about the VisualWorks jitter.

If you like, you can use my test:

100 timesRepeat: [
	| s procs |
	s := Semaphore forMutualExclusion.
	procs := (1 to: 50) collect: [:i |
		[Processor yield. [s critical: []] repeat] forkAt: Processor
activeProcess priority - 1].
	procs do: [:p |
		(Delay forMilliseconds: 2) wait.
		p terminate].
	(Delay forMilliseconds: 100) wait.
	s excessSignals = 0 ifTrue: [self error: 'Failure to release critical
semaphore']].




On Sunday, September 2, 2001, at 11:27 am, Göran Hultgren wrote:

> Hi guys!
>
> I am hacking away on a Flow based client/server thingy and needed to 
> use a Semaphore as a mutex.
> So I did something like this:
>
> myMutex critical: [ ...code code... ^self ...code code...]
>
> ...and oops, got bitten by the fact that the Semaphore does not get 
> signalled when I have a return
> in my critical code block! Of course, it was simple to rewrite the code 
> without returns but...
>
> Is there any particular reason (performance etc) that prevents us from 
> changing the implementation
> of #critical: to this (thus not missing to signal the Semaphore)?:
>
> critical: mutuallyExcludedBlock
> 	"Evaluate mutuallyExcludedBlock only if the receiver is not 
> currently in
> 	the process of running the critical: message. If the receiver is, 
> evaluate
> 	mutuallyExcludedBlock after the other critical: message is finished."
>
> 	self wait.
> 	^mutuallyExcludedBlock ensure: [self signal]
>
>
> The funny thing is that this seems to work EXCEPT for one Unit test I 
> have which seems to "hang"
> on the "self signal" part above. Funny enough, if I alt-. it and then 
> just "proceed" it moves on!
>
> Obviously I am missing some piece of the puzzle, right? In any case 
> this should perhaps be
> mentioned in the comment to #critical:
>
> regards, Gˆran
>
> PS. This is in a rather old image pdated to 2915 with JMM's Flow and 
> SUnit 2.7.1. DS
>
> =====
> Gˆran Hultgren, goran.hultgren at bluefish.se
> GSM: +46 70 3933950, http://www.bluefish.se
> "Department of Redundancy department." -- ThinkGeek
>
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