Hi Eliot,


On 2021-12-28T15:32:13-08:00, eliot.miranda at gmail.com wrote:

> On Tue, Dec 28, 2021 at 2:15 PM Eliot Miranda <eliot.miranda at gmail.com>
> wrote:
> 
> >
> >
> > On Tue, Dec 28, 2021 at 1:53 PM <mail at jaromir.net> wrote:
> >
> >> Hi Eliot, all,
> >>
> >> this example shows Mutex's critical section can be entered multiple times:
> >>
> >
> > I know.  suspend is broken.  Please read my previous message fully and
> > carefully.  If I implement the second alternative then the example works
> > correctly.  In the siulator I get:
> >
> > {a Semaphore(a Process(75019) in [] in [] in UndefinedObject>>DoIt) .
> >  a Mutex(a Process(59775) in Mutex>>critical:) .
> >  a Process(75019) in [] in [] in UndefinedObject>>DoIt . false .
> >  a Process(59775) in Mutex>>critical: . false}
> >
> 
> However, this comes at the cost of finding that the new terminate is
> broken.  If suspend does not remove a process from its list then a process
> terminated while waiting on a Semaphore remains on that semaphore.  So
> terminate must not only ensure that any critical sections are released, but
> that the process is removed from its list, if that list is a condition
> variable.  IMO this should happen very early on in terminate.  I'm running
> the second alternative but I have to filter out unrunnable processes in
> signal et al since suspend no longer removes from the list.
> 
> 

It seems to me #terminate will have to remove the terminating process from any waiting queue in either case - am I right?

At any rate, I think the fix might go like this:

replace:

	oldList := self suspend.

with:

	(oldList := self suspendingList) ifNotNil: [:list | list remove: self ifAbsent:[]. self offList].

Is this sufficient even without self suspend or is calling suspend necessary for something other than removing from the queue? (I don't fully understand the last line of the primitiveSuspend). Self is already suspended when entering this part of the code (either explicitely or by Semaphore/Mutex). By removing it from the waiting queues it should become unawakeable. The next step is indeed setting the suspendedContext to nil.

Many thanks for your advice.

best,
~~~
^[^    Jaromir

Sent from Squeak Inbox Talk
> 
> >>     | s p1 p2 p3 m |
> >>     s := Semaphore new.
> >>     m := Mutex new.
> >>     p1 := [m critical: [s wait]] newProcess.
> >>     p1 resume.
> >>     p2 := [m critical: [s wait]] newProcess.
> >>     p2 resume.
> >>     Processor yield.
> >>     { p1. p1 suspend. p2. p2 suspend }.
> >>     p1 resume. p2 resume.
> >>     Processor yield.
> >>     { s. m. p1. p1 isTerminated. p2. p2 isTerminated. m isOwned. m
> >> instVarNamed: 'owner' }.
> >>     p3 := [m critical: [s wait]] newProcess.
> >>     p3 resume.
> >>     Processor yield.
> >>     { s. m. p1. p1 isTerminated. p2. p2 isBlocked. p3. p3 isBlocked. m
> >> isOwned. m instVarNamed: 'owner' }.
> >>
> >> I've just added a third process to your last example; p3 really enters
> >> the critical section and takes m's ownership despite the fact p2 is already
> >> waiting inside m's  critical section - because p2 managed to enter m
> >> withour taking m's ownership.
> >>
> >> Now we could repeat the procedure and keep adding processes inside the
> >> critical section indefinitely :) So I guess this really is a bug.
> >>
> >> Best,
> >>
> >> ~~~
> >> ^[^    Jaromir
> >>
> >> Sent from Squeak Inbox Talk
> >>
> >> On 2021-12-28T20:07:25+01:00, mail at jaromir.net wrote:
> >>
> >> > Hi Eliot,
> >> >
> >> > Thanks! Please see my comments below, it seems to me there may be a bug
> >> in the Mutex.
> >> >
> >> > ~~~
> >> > ^[^    Jaromir
> >> >
> >> > Sent from Squeak Inbox Talk
> >> >
> >> > On 2021-12-27T14:55:22-08:00, eliot.miranda at gmail.com wrote:
> >> >
> >> > > Hi Jaromir,
> >> > >
> >> > > On Mon, Dec 27, 2021 at 2:52 AM <mail at jaromir.net> wrote:
> >> > >
> >> > > > Hi all,
> >> > > >
> >> > > > What is the desirable semantics of resuming a previously suspended
> >> process?
> >> > > >
> >> > >
> >> > > That a process continue exactly as it had if it had not been
> >> suspended in
> >> > > the first place.  In this regard our suspend is hopelessly broken for
> >> > > processes that are waiting on condition variables. See below.
> >> > >
> >> > >
> >> > > >
> >> > > > #resume's comment says: "Allow the process that the receiver
> >> represents to
> >> > > > continue. Put the receiver in *line to become the activeProcess*."
> >> > > >
> >> > > > The side-effect of this is that a terminating process can get
> >> resumed
> >> > > > (unless suspendedContext is set to nil - see test
> >> KernelTests-jar.417 /
> >> > > > Inbox - which has the unfortunate side-effect of #isTerminated
> >> answer true
> >> > > > during termination).
> >> > > >
> >> > >
> >> > > But a process that is terminating should not be resumable.  This
> >> should be
> >> > > a non-issue.  If a process is terminating itself then it is the active
> >> > > process, it has nil as its suspendedContext, and Processor
> >> > > activeProcess resume always produces an error.. Any process that is
> >> not
> >> > > terminating itself can be made to fail by having the machinery set the
> >> > > suspendedContext to nil.
> >> > >
> >> >
> >> > Yes agreed, but unfortunately that's precisely what is not happening in
> >> the current and previous #terminate and what I'm proposing in
> >> Kernel-jar.1437 - to set the suspendedContext to nil during termination,
> >> even before calling #releaseCriticalSection.
> >> >
> >> > >
> >> > > > A similar side-effect: a process originally waiting on a semaphore
> >> and
> >> > > > then suspended can be resumed into the runnable state and get
> >> scheduled,
> >> > > > effectively escaping the semaphore wait.
> >> > > >
> >> > >
> >> > > Right,  This is the bug.  So for example
> >> > >     | s p |
> >> > >     s *:=* Semaphore new.
> >> > >     p *:=* [s wait] newProcess.
> >> > >     p resume.
> >> > >     Processor yield.
> >> > >     { p. p suspend }
> >> > >
> >> > > answers an Array of process p that is past the wait, and the
> >> semaphore, s.
> >> > > And
> >> > >
> >> > >     | s p |
> >> > >     s *:=* Semaphore new.
> >> > >     p *:=* [s wait] newProcess.
> >> > >     p resume.
> >> > >     Processor yield.
> >> > >     p suspend; resume.
> >> > >     Processor yield.
> >> > >     p isTerminated
> >> > >
> >> > > answers true, whereas in both cases the process should remain waiting
> >> on
> >> > > the semaphore.
> >> > >
> >> > > >
> >> > > > Is this an expected behavior or a bug?
> >> > > >
> >> > >
> >> > > IMO it is a dreadful bug.
> >> > >
> >> > > > If a bug, should a suspended process somehow remember its previous
> >> state
> >> > > > and/or queue and return to the same one if resumed?
> >> > > >
> >> > >
> >> > > IMO the primitive should back up the process to the
> >> > > wait/primitiveEnterCriticalSection. This is trivial to implement in
> >> the
> >> > > image, but is potentially non-atomic.  It is perhaps tricky to
> >> implement in
> >> > > the VM, but will be atomic.
> >> > >
> >> > > Sorry if I'm missing something :)
> >> > > >
> >> > >
> >> > > You're not missing anything :-)  Here's another example that answers
> >> two
> >> > > processes which should both block but if resumed both make progress.
> >> > >
> >> > >     | s p1 p2 m |
> >> > >     s *:=* Semaphore new.
> >> > >     m *:=* Mutex new.
> >> > >     p1 *:=* [m critical: [s wait]] newProcess.
> >> > >     p1 resume.
> >> > >     p2 *:=* [m critical: [s wait]] newProcess.
> >> > >     p2 resume.
> >> > >     Processor yield.
> >> > >     { p1. p1 suspend. p2. p2 suspend }
> >> > >
> >> > > p1 enters the mutex's critical section, becoming the mutex's owner.
> >> p2 then
> >> > > blocks attempting to enter m's critical section.  Let's resume these
> >> two,
> >> > > and examine the semaphore and mutex:
> >> > >
> >> > >     | s p1 p2 m |
> >> > >     s *:=* Semaphore new.
> >> > >     m *:=* Mutex new.
> >> > >     p1 *:=* [m critical: [s wait]] newProcess.
> >> > >     p1 resume.
> >> > >     p2 *:=* [m critical: [s wait]] newProcess.
> >> > >     p2 resume.
> >> > >     Processor yield.
> >> > >     { p1. p1 suspend. p2. p2 suspend }.
> >> > >     p1 resume. p2 resume.
> >> > >     Processor yield.
> >> > >     { s. m. p1. p1 isTerminated. p2. p2 isTerminated }
> >> > >
> >> > > In this case the end result for p2 is accidentally correct. It ends up
> >> > > waiting on s within m's critical section. But p1 ends up terminated.
> >> IMO
> >> > > the correct result is that p1 remains waiting on s, and is still the
> >> owner
> >> > > of m, and p2 remains blocked trying to take ownership of m.
> >> > >
> >> >
> >> > Perfect example! My naive expectation was when a process inside a
> >> critical section gets suspended the Mutex gets unlocked but that's
> >> apparently wrong :)
> >> >
> >> > But still, there's something wrong with the example: If p1 resumes it
> >> releases m's ownership and terminates, then p2 takes over and proceeds
> >> inside the critical section and gets blocked at the semaphore. I'd expect
> >> p2 would become the owner of the Mutex m BUT it's not! There's no owner
> >> while p2 is sitting at the semaphore. Try:
> >> >
> >> >     | s p1 p2 m |
> >> >     s := Semaphore new.
> >> >     m := Mutex new.
> >> >     p1 := [m critical: [s wait]] newProcess.
> >> >     p1 resume.
> >> >     p2 := [m critical: [s wait]] newProcess.
> >> >     p2 resume.
> >> >     Processor yield.
> >> >     { p1. p1 suspend. p2. p2 suspend }.
> >> >     p1 resume. p2 resume.
> >> >     Processor yield.
> >> >     { s. m. p1. p1 isTerminated. p2. p2 isTerminated. m isOwned. m
> >> instVarNamed: 'owner' }
> >> >
> >> > It seems to me that when p2 gets suspended it is stopped somewhere
> >> inside #primitiveEnterCriticalSection before the owner is set and when it
> >> gets resumed it is placed into the runnable queue with the pc pointing
> >> right behind the primitive and so when it runs it just continues inside
> >> #critical and get blocked at the semaphore, all without having the
> >> ownership.
> >> >
> >> > Is this interpretation right? It would mean Mutex's critical section
> >> can be entered twice via this mechanism...
> >> >
> >> > Cuis does set the ownership to p2 in this example.
> >> >
> >> > Thanks again,
> >> >
> >> > Jaromir
> >> > >
> >> > > >
> >> > > > Best,
> >> > > > ~~~
> >> > > > ^[^    Jaromir
> >> > > >
> >> > > > Sent from Squeak Inbox Talk
> >> > > >
> >> > >
> >> > > _,,,^..^,,,_
> >> > > best, Eliot
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> >> > >
> >> >
> >> >
> >>
> >
> >
> > --
> > _,,,^..^,,,_
> > best, Eliot
> >
> 
> 
> -- 
> _,,,^..^,,,_
> best, Eliot
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