Blocking a task indefinitely in the RTS

[Phyx]

I have simply copy pasted the code you provided. Note that my actual code
code doesn't pass anything to a foreign interface. It stores everything in
a Haskell mutable object.
The RTS on completion of actions simply schedules a task which inspects
this objects and wakes up as many blocked tasks as possible.

> Strange, how could the scheduler assume a deadlock if the MVar could be
called from a closure that is still alive?  Can you show the code that
you're testing?

Because as far as I can tell, it doesn't care. When it comes to MVars and
STMs the scheduler assumes a deadlock when all tasks on all capabilities
are blocked.
For the threaded runtime it has an early exit condition from this code in
the case where there has been any activity in a complete timeslice or when
you're blocked on
specific calls such as I/O.

On the non-threaded runtime the timeslice case doesn't apply and you only
have one capability, it will force a GC to try to revive some tasks, and if
at the end of
this the tasks are still blocked it will release one in order to attempt to
proceed. In short, as far as I can tell I don't think it considers
reach-ability at all, not for MVars.

This is why things e.g. doing a takeMVar will also process pending puts
etc, so that if you actually enter a blocked state, you know you had no
other choice.

On Mon, Jan 7, 2019 at 11:24 PM Phil Ruffwind <rf at rufflewind.com> wrote:

> Strange, how could the scheduler assume a deadlock if the MVar could be
> called from a closure that is still alive?  Can you show the code that
> you're testing?
>
> On Mon, Jan 7, 2019, at 14:09, Phyx wrote:
> > Hi Phil,
> >
> > Thanks for the reply, however that just gives me a forced deadlock
> removal
> > as before.
> >
> > new bound thread (1)
> > cap 0: schedule()
> > cap 0: running thread 1 (ThreadRunGHC)
> > cap 0: thread 1 stopped (blocked on an MVar)
> >         thread    1 @ 0000000003205388 is blocked on an MVar @
> > 00000000032040c8 (TSO_DIRTY)
> > deadlocked, forcing major GC...
> > all threads:
> > threads on capability 0:
> > other threads:
> >         thread    1 @ 0000000003205388 is blocked on an MVar @
> > 00000000032040c8 (TSO_DIRTY)
> > cap 0: starting GC
> >
> > I don't believe any solution involving MVars will work for the
> non-threaded
> > RTS. Though I'd love to be wrong here...
> >
> > Regards,
> > Tamar
>
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