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

[Rob Hoes <Rob.Hoes@xxxxxxxxxx>]

> On Mon, 2013-12-02 at 11:58 +0000, Rob Hoes wrote:
> > > > > Can't you just use caml_register_global_root on the address of
> > > > > your value for_callback? I presume you can call this
> > > > > register/unregister pair many times (for different events etc)?
> > > >
> > > > I think you need to register a value, not a pointer to a value.
> > >
> > > CAMLextern void caml_register_global_root (value *);
> > >
> > > It takes a pointer to the value to register. Is there any reason why
> > > you can't pass &for_callback given "value for_callback" in the local
> context?
> >
> > I think we'd then have to rely on libxl to never copy to pointer
> > internally, and throw away the original one. I am worried that this
> > may happen, because the ao_how value is temporary.
> 
> libxl will for certain take a copy of the pointer passed into the ao_how,
> because it stashes it for later use.
> 
> Either I'm very confused about what you are suggesting or pointers in C
> don't work like you seem to think.
> 
> AFAICT caml_register_global_root takes a pointer to the value and locks
> down the value itself, preventing it from moving. There is no way for it
> to do anything else given a pointer and not a pointer to a pointer.

That's the point, I don't think that is how it works.

I think the value on the heap can still move around, but registering a pointer 
to it as a root means that:
1. the value does not get dropped by the GC;
2. the root/pointer will be updated by the GC when the value itself moves, so 
that you can still find it back.

Rob

> Ian.
> 
> >
> > > >
> > > > > Otherwise you need to build some sort of C-side data structure
> > > > > to hold all the various values, which is doable but a bit
> > > > > tedious I
> > > imagine.
> > > >
> > > > I think we can malloc a new "value" (pointer to ocaml heap), make
> > > > it equal to the for_callback value, register this with the GC (so
> > > > it can update this heap pointer when the actual value moves), and
> > > > give a pointer to it to libxl. And then of course deregister and
> > > > free before we give it back to ocaml.
> > >
> > > I think this (moving the heap pointer from the GC) would require the
> > > prototype to be "value **" not "value *".
> > >
> > > Ian.
> >
> 

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