[Caml-list] Nested callbacks?

[Caml-list] Nested callbacks?

[Dmitry Bely]

Is it allowed to call a Caml closure from C (caml_callbackN_exn), that
calls another Caml closure internally (also with caml_callbackN_exn)?

- Dmitry Bely

Re: [Caml-list] Nested callbacks?

[Guillaume Yziquel]

Le Tuesday 05 Jul 2011 à 20:06:58 (+0400), Dmitry Bely a écrit :
> Is it allowed to call a Caml closure from C (caml_callbackN_exn), that
> calls another Caml closure internally (also with caml_callbackN_exn)?
> 
> - Dmitry Bely

I do not see why not. Not 100% sure, but I believe some code of mine did
that.

-- 
     Guillaume Yziquel

Re: [Caml-list] Nested callbacks?

[Thomas Fischbacher]

Dmitry Bely wrote:

> Is it allowed to call a Caml closure from C (caml_callbackN_exn), that
> calls another Caml closure internally (also with caml_callbackN_exn)?

I strongly hope so! If this did not work, that would have disastrous
consequences for the tight integration of Caml and Python which we are
using. (Well, so far, we never encountered a problem with that. And the
documentation does not warn against doing this - so, should it not work,
that should be considered a bug.)

-- 
best regards,
Thomas Fischbacher
t.fischbacher@soton.ac.uk

Re: [Caml-list] Nested callbacks?

[Xavier Leroy]

On 07/05/2011 06:30 PM, Thomas Fischbacher wrote:
> 
> Dmitry Bely wrote:
> 
>> Is it allowed to call a Caml closure from C (caml_callbackN_exn), that
>> calls another Caml closure internally (also with caml_callbackN_exn)?
> 
> I strongly hope so! If this did not work, that would have disastrous
> consequences for the tight integration of Caml and Python which we are
> using. (Well, so far, we never encountered a problem with that. And the
> documentation does not warn against doing this - so, should it not work,
> that should be considered a bug.)

Indeed, it should work, and I see no reason why nested callbacks could
fail.  Callbacks do save some Caml-specific state and restore it
before returning, but they use the stack to do so, so they should be
reentrant.  Please file a bug report if you find out they are not.

- Xavier Leroy

Re: [Caml-list] Nested callbacks?

[Dmitry Bely]

On Tue, Jul 5, 2011 at 9:31 PM, Xavier Leroy <Xavier.Leroy@inria.fr> wrote:
> On 07/05/2011 06:30 PM, Thomas Fischbacher wrote:
>>
>> Dmitry Bely wrote:
>>
>>> Is it allowed to call a Caml closure from C (caml_callbackN_exn), that
>>> calls another Caml closure internally (also with caml_callbackN_exn)?
>>
>> I strongly hope so! If this did not work, that would have disastrous
>> consequences for the tight integration of Caml and Python which we are
>> using. (Well, so far, we never encountered a problem with that. And the
>> documentation does not warn against doing this - so, should it not work,
>> that should be considered a bug.)
>
> Indeed, it should work, and I see no reason why nested callbacks could
> fail.  Callbacks do save some Caml-specific state and restore it
> before returning, but they use the stack to do so, so they should be
> reentrant.  Please file a bug report if you find out they are not.

Thanks. I am trying to find a memory bug in a multithreaded
application that calls bytecode Ocaml runtime from C via callbacks. I
was able to extract a test that triggers the bug. Actually it's just
an endless loop with a single callback inside that only allocates some
memory on the Ocaml heap. Interesting enough that when I have only one
Caml thread (and no Caml tick thread running), everything goes OK. But
when I register 2 threads in Ocaml runtime (one worker, one always
sleeping), thus enabling the tick thread and related preempt signal
machinery, after many cycles I get a memory bug in the worker thread:

Starting new major GC cycle
### O'Caml runtime: heap check ###
file freelist.c; line 397 ### Assertion failed: prev < bp || prev == Fl_head

(Assertion is failed because prev == bp)

As the preempt signal is also processed via Caml callback, I thought
the problem might be nested callbacks. But if they are OK - well, I
just have to continue debugging. If I find what goes wrong, of course
I will file a bug report.

- Dmitry Bely

Re: [Caml-list] Nested callbacks?

[rixed]

> file freelist.c; line 397 ### Assertion failed: prev < bp || prev == Fl_head

I stumbled upon this one when using threads on a program of mine some
time ago on ARM arch (but not others). Do you run your code on ARM by
any chance?

Re: [Caml-list] Nested callbacks?

[Dmitry Bely]

On Tue, Jul 5, 2011 at 10:06 PM,  <rixed@happyleptic.org> wrote:
>> file freelist.c; line 397 ### Assertion failed: prev < bp || prev == Fl_head
>
> I stumbled upon this one when using threads on a program of mine some
> time ago on ARM arch (but not others). Do you run your code on ARM by
> any chance?

No, it's x86.

- Dmitry Bely

Re: [Caml-list] Nested callbacks?

[Dmitry Bely]

Of course I have found my own bug and nested callbacks work as
expected. Sorry about the noise in the list.

- Dmitry Bely