[Vm-dev] [BUG] Mysterious Delay lockups

Andreas Raab andreas.raab at gmx.de
Wed Apr 18 02:20:28 UTC 2007

Hi Folks -

Some of you (mostly those who run heavy servers) may have noticed that 
at times Squeak locks up in mysterious and unforeseen ways. One of those 
lockups involves Delay's AccessProtect in an unsignaled state and 
consequently the entire image locking up since Delay access is required 
in many, many places.

Today, David presented me an image that was locked up in such a state 
but by sheer luck he managed to save it right before it happened which 
allowed me to investigate the situation. The result can best be 
explained by the little test case shown here:

   "Create mutex unsignaled so we can manually signal it"
   mutex := Semaphore new.
   "Create a process which will wait inside the mutex"
   p := [mutex critical:[]] forkAt: Processor userBackgroundPriority.
   "Wait until process has entered mutex"
   [p suspendingList == mutex]
       whileFalse:[(Delay forMilliseconds: 10) wait].
   "Signal mutex"
   mutex signal.
   "Kill process"
   p terminate.
   "and check to see if the mutex is signaled"
   mutex isSignaled ifFalse:[self error: 'Mutex not signaled'].

Note that despite the somewhat complex setup the basic idea is that a 
low priority process waiting in a critical section receives a signal on 
the semaphore it is waiting on but gets terminated by a higher priority 
process inbetween receiving the signal and execution of the process itself.

This situation (manually executed in the above to make it more easily 
repeatable) can happen in many situations where processes get terminated 
"from the outside" and it would cause particular grief in the timing 
semaphore because it gets served by the highest priority process which 
makes the unfortunate cause of events much more likely.

All Squeak versions that I have access to expose this behavior. Looking 
at Semaphore>>critical: which says

Semaphore>>critical: aBlock
   self wait.
   [blockValue := aBlock value] ensure: [self signal].

makes it seem as if moving the wait into the ensured block is the 
correct answer, but that ain't necessarily so. When we move the wait 
into the block we risk that the entering process is terminated after 
entering the block but before entering the wait which would leave the 
semaphore signaled twice, which is just as bad as not signaled at all.

Methinks a solution would involve Process>>terminate but I'm running out 
of steam after trying to understand the problem in all its implications. 
Any ideas would be greatly welcome.

   - Andreas

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