Some more digging revealed a pending VW fix that might be related - it
purports to fix a hole in Semaphore>>critical: and I wonder whether it
might apply to Squeak as well.

Current Squeak implementation:
critical: mutuallyExcludedBlock
	| blockValue |
	self wait.
	[blockValue _ mutuallyExcludedBlock value]
		ensure: [self signal].
	^blockValue

Current VisualWorks implementation:
critical: mutuallyExcludedBlock
	self waitIfCurtailedSignal.
	^mutuallyExcludedBlock
		ensure: [self signal]

Proposed fix (see http://tinyurl.com/9ed25):
critical: mutuallyExcludedBlock
       | recover |
        recover := false.
        ^[
                | result |
                self waitIfCurtailedSignal.
                "The dynamic translator creates code that can't be
interrupted here (I think)."
                recover := true.
                result := mutuallyExcludedBlock value.
                recover := false.
                "The dynamic translator creates code that can't be
interrupted here (I think)."
                self signal.
                result
        ] ifCurtailed: [
                recover ifTrue: [
                        recover := false.
                        self signal]].

The fix has some test code in its comment (again, see the tinyurl)
that I'm going to run now, but I was wondering whether people could
comment on the applicability to Squeak of this fix. In VW, it seems to
make a difference...

On 1/5/06, Cees De Groot <cdegroot at gmail.com> wrote:
> Below a stack trace we've been seeing a couple of times lately in our
> wxSqueak project (which is mostly 3.8). It seems that SuspendedDelays
> has a nil element somewhere. However, the code in Delay seems to be
> safe against that and I can't see any code elsewhere in the image that
> messes around with delays...
>
> Has anyone ever seen this?
>
> UndefinedObject(Object)>>doesNotUnderstand: #resumptionTime
> [] in Delay class>>DoIt {[:t2 :t3 | t2 resumptionTime <= t3 resumptionTime]}
> SortedCollection>>indexForInserting:
> SortedCollection>>add:
> [] in Delay>>schedule {[beingWaitedOn := true.  resumptionTime := Time
> millisecondClockValue + dela...]}
> [] in Semaphore>>critical: {[t2 := t1 value]}
> BlockContext>>ensure:
> Semaphore>>critical:
> Delay>>schedule
> Delay>>wait
> TUDispatchingDatagramChannel(TUDatagramChannel)>>retransmissionRun
> [] in TUDispatchingDatagramChannel(TUDatagramChannel)>>start {[self
> retransmissionRun]}
> BlockContext>>repeat
> [] in TUDispatchingDatagramChannel(TUDatagramChannel)>>start {[[self
> retransmissionRun] repeat]}
> [] in BlockContext>>newProcess {[self value.  Processor terminateActive]}
>