[squeak-dev] SIGTRAP when Proceeding from BlockCannotReturn

Jaromir Matas mail at jaromir.net
Tue May 10 20:36:25 UTC 2022


Hi Eliot,

Thanks a lot for your answers.

One last question: what about Process >> #signalException:? You wrote a fix (enclosed) – is it ok to merge it when the #suspend method is updated (Nicolas’s way)?

Thanks again,
Jaromir


--

Jaromír Matas

+420 777 492 777
mail at jaromir.net

From: Eliot Miranda<mailto:eliot.miranda at gmail.com>
Sent: Tuesday, May 10, 2022 21:17
To: The general-purpose Squeak developers list<mailto:squeak-dev at lists.squeakfoundation.org>
Subject: Re: [squeak-dev] SIGTRAP when Proceeding from BlockCannotReturn

Hi Jaromir,

On Tue, May 10, 2022 at 8:31 AM Jaromir Matas <mail at jaromir.net<mailto:mail at jaromir.net>> wrote:
Hi Eliot, Nicolas,

> > > (Eliot) Apologies for not responding sooner.  It seems to me the semantics must remain as they are, and it be left up to the image to bind suspend to the primitive most convenient. Then backwards compatibility can be provided for cases such as DelayWaitTimeout by providing a renamed suspend which accesses the old primitive.
> >
> > (Nicolas) Yes, I propose the selector
> Process>>suspendAndUnblock <primitive: 78> snip...
> Process>>suspend would call the new primitive, and fallback to suspendAndUnblock in case of failure (for example if new primitive is absent).
> The question remaining is if we really need two new primitives 578 (V1) and 588 (V2).
> It seems like signalException: would require V1, while Jaromir terminate logic would require V2...
>
> (Eliot) +1. Works for me.  Having both 578 & 588 available in the VM is fine. I agree that surfacing only one of them is best.

Thank you, I wasn't sure how to proceed with this :) Do I understand correctly #suspend will have the new suspend semantics (either V1 or preferably V2) in new images (new release) and the previous semantics (via prim 88) will be available for backward compatibility (or special usage)?

Yes, exactly.
In that case the tests I submitted in KernelTests-jar.421 should work without changes...

Cool!

The next question is what to do with:
Process >> suspendPrimitivelyOrFail
                "Test support. Execute primitive 88, or fail."

                <primitive: 88>
                ^self primitiveFailed

Change it or remove it? Replace with Nicolas’s Process>>suspendAndUnblock ?

Well it appears to be used only in testAtomicSuspend.  So I would change it to use the same primitive number as suspend, which I guess is 578, primitiveSuspendBackingUpV2, which has the more consistent semantics.


Thanks!

Best,

Jaromir

From: Eliot Miranda<mailto:eliot.miranda at gmail.com>
Sent: Monday, May 9, 2022 23:23
To: The general-purpose Squeak developers list<mailto:squeak-dev at lists.squeakfoundation.org>
Subject: Re: [squeak-dev] SIGTRAP when Proceeding from BlockCannotReturn



On May 9, 2022, at 1:04 PM, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com<mailto:nicolas.cellier.aka.nice at gmail.com>> wrote:


Hi Eliot,
Le lun. 9 mai 2022 à 20:36, Eliot Miranda <eliot.miranda at gmail.com<mailto:eliot.miranda at gmail.com>> a écrit :
Hi Jaromir, Hi Nicolas,

On Sun, May 8, 2022 at 8:17 AM Jaromir Matas <mail at jaromir.net<mailto:mail at jaromir.net>> wrote:
Hi Nicolas,

>> About the latest VM, (Smalltalk processSuspensionUnblocks) answers about the behavior of primitiveSuspend (#88).
> Err,  wrong wording (Smalltalk processSuspensionUnblocks) does not describe the behavior of primitiveSuspend #(88).
> It answers whether the VM has primitives #578 and #588 (see #getCogVMFeatureFlags in VMMaker)...
>
Yes, thanks, well that was the original idea for Smalltalk processSuspensionUnblocks  to answer about the behavior of the primitiveSuspend #88; which was before the primitives #568 and #578 were introduced. (And which was when I wrote the tests using this original logic). However that approach ran into problems with some existing implementations (e.g. Virtend) and as a result the two new primitives were introduced (#568 and #578) - and apparently the logic of what Smalltalk processSuspensionUnblocks answers about changed. Since then I haven't heard from Eliot about his plans how to proceed with the new suspend semantics or whether this is it :)

Apologies for not responding sooner.  It seems to me the semantics must remain as they are, and it be left up to the image to bind suspend to the primitive most convenient. Then backwards compatibility can be provided for cases such as DelayWaitTimeout by providing a renamed suspend which accesses the old primitive.
Yes, I propose the selector
Process>>suspendAndUnblock <primitive: 78> snip...
Process>>suspend would call the new primitive, and fallback to suspendAndUnblock in case of failure (for example if new primitive is absent).
The question remaining is if we really need two new primitives 578 (V1) and 588 (V2).
It seems like signalException: would require V1, while Jaromir terminate logic would require V2...

+1. Works for me.  Having both 578 & 588 available in the VM is fine. I agree that surfacing only one of them is best.


Sorry about not renaming processSuspensionUnblocks.  This should be something like Smalltalk hasExtendedProcessSuspensionSemantics or some such.

It's not too late, there are not many usages, except inbox experimentations.




In case this is still WIP I'd quite like an approach similar to Smalltalk processPreemptionYields - you'd set Smalltalk processSuspensionUnblocks true or false depending on how you want the VM to behave and the VM would use the right semantics (unless this is too naive :) ).

Regarding the tests failing with the newest VM:
testAtomicSuspend - we can remove this updated version and leave the existing one for the moment
testRevisedSuspendExpectations - we can leave this one out too
testTerminateBlockedInNestedEnsure1 - I'll take a look at these two and try to adjust the logic
testTerminateBlockedInNestedEnsure2

All remaining test in KernelTests-jar.421 work as intended.

Thanks again,
Jaromir


--

Jaromír Matas

mail at jaromir.net<mailto:mail at jaromir.net>

From: Nicolas Cellier<mailto:nicolas.cellier.aka.nice at gmail.com>
Sent: Sunday, May 8, 2022 16:22
To: The general-purpose Squeak developers list<mailto:squeak-dev at lists.squeakfoundation.org>
Subject: Re: [squeak-dev] SIGTRAP when Proceeding from BlockCannotReturn



Le dim. 8 mai 2022 à 15:59, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com<mailto:nicolas.cellier.aka.nice at gmail.com>> a écrit :


Le dim. 8 mai 2022 à 14:16, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com<mailto:nicolas.cellier.aka.nice at gmail.com>> a écrit :
Hi Jaromir

Le sam. 7 mai 2022 à 14:45, Jaromir Matas <mail at jaromir.net<mailto:mail at jaromir.net>> a écrit :
Hi Marcel, Nicolas, Eliot,


> (Nicolas) I have merged Kernel-jar.1446 which is trivial (re-signal an Exception).

Thanks!

> (Marcel) Hmm... #testSimpleResignalVsOuter1 is still failing.

In my latest trunk image it passes ok (5/7/22)

>> (Marcel) I will also take a look at KernelTests-jar.421 to check whether any new semantics are okay.
>>
> (Nicolas) Yes, I did that and got two failing tests
> testTerminateTerminatingProcess
> testResumeTerminatingProcess
>
> both failures look the same, in second self assert: terminator isSuspended.

Yes, they are supposed to fail at the moment so I suggest to make them expected failures/feature requests. However there's a stupid bug in both I’ve fixed now: they were indeed supposed to fail the following assertion:
``` self should: [terminatee terminate] raise: Error. ```

Apologies for the confusion - an updated changeset is enclosed.


However, there's another issue regarding the revised #suspend semantics Eliot has been working on. I've updated the process tests in KernelTests-jar.421 to test both the old and the new #suspend semantics. The two semantics should be distinguishable via Smalltalk processSuspensionUnblocks flag answering true in case of the old semantics and false in case of the revised one; my updated tests use this logic. However, unfortunately the latest VM answers "false" but uses the OLD suspend semantics in #suspend prim 88.

So I'm surprised you haven't observed more tests failing due to the new suspend semantics... What VM have you used – an older one? I'm on the latest trunk's VM 3183. And what is the answer of Smalltalk processSuspensionUnblocks? :) I'm utterly confused...

Hi Eliot - I may be confused here but if the current VM uses the old #suspend prim 88 semantics, shouldn't ```Smalltalk processSuspensionUnblocks``` answer true?


Indeed, I can confirm that the latest VM does answer false to (Smalltalk processSuspensionUnblocks).
Though, process suspension still unblocks as shown by the example at
https://github.com/pharo-project/pharo/issues/10669

A bit more simply, this answers false - it shouldn't:

s := Semaphore new.
p := [s wait] newProcess.
p resume.
100 milliSeconds wait.
p suspend; resume.
100 milliSeconds wait.
p isTerminated = Smalltalk processSuspensionUnblocks.

technically, this answers false - it shouldn't:

s := Semaphore new.
p := [s wait] newProcess.
p resume.
100 milliSeconds wait.
p suspend == s = Smalltalk processSuspensionUnblocks


About the latest VM, (Smalltalk processSuspensionUnblocks) answers about the behavior of primitiveSuspend (#88).
Err,  wrong wording (Smalltalk processSuspensionUnblocks) does not describe the behavior of primitiveSuspend #(88).
It answers whether the VM has primitives #578 and #588 (see #getCogVMFeatureFlags in VMMaker)...

The new Behavior is implemented by primitiveSuspendBackingUpV1 (#578) and primitiveSuspendBackingUpV2 (#588).
V1 always answers the old list (even if a Semaphore/Mutex), V2 answers nil in case of Semaphore/Mutex.

So, only ((Process>>#suspend) primitive) can give you a clue about the behavior, assuming that the primitive in question is implemented...

So it depends if we want to make it a Preference, or make the image compatible with some less capable VM...

we could implement
    Process>>primitiveSuspendAndUnblock as <primitive: 88>
    Process>>primitiveSuspend as <primitive: 588>
test the VM flag in suspend
    Process>>suspend
        ^(VMHasPrimitiveSuspendBackingUp ifNil: [VMHasPrimitiveSuspendBackingUp := Smalltalk processSuspensionUnblocks not])
            ifTrue: [self primitiveSuspend]
            ifFalse: [self primitiveSuspendAndUnblock]
Then arrange to reset VMHasPrimitiveSuspendBackingUp to nil at snapshot...

I will tell next week about the status of the VM which I used for testing, it's possibly a few months old.

> (Nicolas) I would also consider 1445 1421 and most importantly 1447.

Regarding 1421: alternatively, you might consider a newer version 1415 where the only difference is #return:from: passes a block instead of nil to #resume:through: to cause a fresh search for the first unwind context; otherwise they are equivalent.

OK, I will check again.

> (Marcel) Maybe we can just merge
>
> KernelTests-jar.393
> KernelTests-jar.418
> KernelTests-jar.421
>
> And go from there? Or are there any objections?

KernelTests-jar.393 is just a special case of a more general KernelTests-jar.418 test…

In my image all tests are green provided Smalltalk processSuspensionUnblocks answers true and the two above tests Nicolas mentioned are marked expected failures :)

Thanks very much for your feedback,
Jaromir


--

Jaromír Matas

mail at jaromir.net<mailto:mail at jaromir.net>





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best, Eliot






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best, Eliot

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