[M3devel] CM3 crashing

Peter McKinna peter.mckinna at gmail.com
Mon Jan 13 04:54:08 CET 2014


  I was trying to get a handle on that problem last year. The threadtest
program is really a stress tester of the collector/allocator with pthreads.
If you run it with just the tests read and alloc you pretty much always get
a crash. If you run them with paranoidgc it will crash in the heap checker.
Tony thought it was a clear cut  problem of the roots of some ref not being
found on a thread stack. I mucked around with code to get some output and
the stacks looked ok to me but I could be wrong. All stacks are checked
whilst the threads are blocked in a signal handler, the design of which
looks fine as far as I can tell. This test is characterised by some slow
threads (the read threads) and a bunch of fast threads (the alloc threads).
Even if you modify the test to have only one read thread the problem
occurs. I have had misgivings about mixing signals and threads having been
bitten many years ago, but really this is the only way the collector can
get its raw refs to check.
My guess is its a subtle timing or lock problem maybe a lurking bug in the
collector itself. One thing I noticed is that in
ThreadPThread__ProcessStopped  the second call to   p(context, ((char
*)context) + sizeof(ucontext_t)); according to the comment is to process
the registers. But the registers should already be on the stack and anyway
this call is a partial duplicate of the previous one.
It would be good to raise the priority on this problem. Trust in the
collector has always been at the heart of m3 programs.

Regards Peter

On Mon, Jan 13, 2014 at 11:25 AM, <mika at async.caltech.edu> wrote:

> Yes it works in PM3 (still, since I use PM3 on FreeBSD4, never saw a
> reason to switch to CM3).
> I figured there aren't actually implementations of Word.Xor, Word.Or,
> and Word.And as procedures but they get inlined somehow?
> BTW, any idea about what's wrong with pthreads?  Do you think the issue
> is with the garbage collector or with the threads, off the top of your
> head?
> Tony Hosking writes:
> >Are you saying passing these used to work in PM3?
> >Sounds like a front-end bug.  I=92m curious what changed to break it.
> >
> >On Jan 12, 2014, at 12:58 PM, mika at async.caltech.edu wrote:
> >
> >>=20
> >> The code is:
> >>=20
> >>              HIntExpr.Xor =3D> RETURN NewConst(CBitwise(av, bv, =
> >Word.Xor), ab)
> >>            |
> >>              HIntExpr.Bor =3D> RETURN NewConst(CBitwise(av, bv, =
> >Word.Or), ab)
> >>            |
> >>              HIntExpr.Band =3D> RETURN NewConst(CBitwise(av, bv, =
> >Word.And), ab)
> >>=20
> >> I guess it doesn't like passing Word.Xor, Word.Or, and Word.And ...=20
> >
> >
> >
> >Antony Hosking | Associate Professor | Computer Science | Purdue =
> >University
> >305 N. University Street | West Lafayette | IN 47907 | USA
> >Mobile +1 765 427 5484
> >
> >
> >
> >
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