Re: [Xen-devel] [PATCH v6 08/12] libxl: ocaml: drop the ocaml heap lock before calling into libxl

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-11-28 at 18:11 +0000, Ian Jackson wrote:
> Rob Hoes writes ("RE: [PATCH v6 08/12] libxl: ocaml: drop the ocaml heap lock 
> before calling into libxl"):
> > Ian Jackson wrote:
> > > I don't see here how "async" is protected from being gc'd (or, perhaps,
> > > moved by the ocaml gc) during the execution of the long-running libxl
> > > operation.  The pointer to it is hidden inside the (now malloc'd) ao_how.
> > > AIUI the "CAMLparam1...CAMLreturnT" pair protect it during the aohow_val
> > > function ?
> > 
> > To keep the async user values "alive", we keep them in a set in
> > OCaml for the duration of the async call. See the code that deals
> > with "async_users" in xenlight.ml.in. The GC will definitely not
> > destroy these values.
> 
> Surely that can't be right.  You're relying on the ocaml code to do
> the memory management right.  Effectively you've made an unsafe
> interface!
> 
> > I'm not sure if the GC may _move_ the value (I assumed it wouldn't)...
> 
> I thought GCs in these kind of languages were entitled to move things
> to which they thought they had all the references.

Me too, and the ocaml books explicitly say that it will, e.g.
http://caml.inria.fr/pub/docs/oreilly-book/html/book-ora116.html
        Any allocation in the Objective CAML heap can trigger a garbage
        collection, which will deallocate unused memory blocks and may
        move live blocks.

Does the gc have some sort of horror mode. where it would move stuff
around at every allocation etc? Perhaps a debug option here to always
manually invoke the gc when you acquire/drop the lock and in other key
places?

I wonder if you need to use these functions:
> /* [caml_register_global_root] registers a global C variable as a memory root
>    for the duration of the program, or until [caml_remove_global_root] is
>    called. */
> 
> CAMLextern void caml_register_global_root (value *);
> 
> /* [caml_remove_global_root] removes a memory root registered on a global C
>    variable with [caml_register_global_root]. */
> 
> CAMLextern void caml_remove_global_root (value *);

To keep the for_callback value live?

Ian.


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