[Vm-dev] [VM-dev] Terminated process with zero pc

Eliot Miranda eliot.miranda at gmail.com
Tue May 5 19:52:35 UTC 2020


Hi Denis,

> On May 5, 2020, at 12:39 PM, Denis Kudriashov <dionisiydk at gmail.com> wrote:
> 
> 
> Hi.
> 
> I encounter interesting behavior of process termination. I can't reproduce it locally but I have constantly failing test on Pharo CI (as part of full test suite) due to following conditions:
> 
> ended := false.
> process := [ Processor activeProcess suspend. ended := true] 
>                        forkAt: Processor activePriority + 1.
> self assert: process isSuspended description: 'should be suspended'.
> 
> process resume.
> 
> self assert: ended description: 'last statement is done'.
> self assert: process suspendedContext pc equals: 0
> 
> When I try to run it locally the pc is always greater than #startpc. 
> On CI the test very rarely behaves same way. The process is always finished (#ended is true) but in most of runs the pc of #terminate context is somehow reset to zero (probably related to the order of overall test suite). . 
> 
> In Pharo succeed process ends here:
> 
> terminate
>             ...........
>  self isActiveProcess ifTrue: [
> thisContext terminateTo: nil.
> self suspend ]
> 
> It seems that Context>>#terminateTo: can reset the pc to zero under some condition (jitter related?).

The JIT maps machine code PCs to bytecode PCs whenever the pc is accessed of a context which is running JITted code.  If the mapping fails then 0 may be returned.  But this is evidence of a bug in the mapping computation.

> So I wonder what could be the explanation?
> 
> I would like to understand this behavior before I will propose the change for Pharo (#isTerminate implementation needs to be fixed).

There are exhaustive tests which test all pairs of mappable PCs map correctly in VMMaker.oscog.  Those tests could be run against all methods in Pharo.  The tests use in-image compilation to produce the JITted methods.

But if there is a if such that either the tests are deficient or the VM is somehow suspending at a non-suspension point, the tests will not shoe the problem.

I would work in trying to reproduce the problem, especially in an assert-enabled vm run on the command line.  Then you will see issues in the mapping algorithm well before the mapping algorithm fails and answers a zero pc.

> 
> Best regards,
> Denis.
> 
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