question about the GC implementation

ryles rylesny at gmail.com
Sat Oct 3 23:58:39 CEST 2009


On Oct 2, 11:20 am, Francis Moreau <francis.m... at gmail.com> wrote:
> Hello,
>
> I'm looking at gcmodule.c and in move_unreachable() function, the code
> assumes that if an object has its gc.gc_refs stuff to 0 then it *may*
> be unreachable.
>
> How can an object tagged as unreachable could suddenly become
> reachable later ?
>
> Thanks

It looks like you're not reading through all of the comments:

GC_TENTATIVELY_UNREACHABLE
    move_unreachable() then moves objects not reachable (whether
directly or
    indirectly) from outside the generation into an "unreachable" set.
    Objects that are found to be reachable have gc_refs set to
GC_REACHABLE
    again.  Objects that are found to be unreachable have gc_refs set
to
    GC_TENTATIVELY_UNREACHABLE.  It's "tentatively" because the pass
doing
    this can't be sure until it ends, and GC_TENTATIVELY_UNREACHABLE
may
    transition back to GC_REACHABLE.

    Only objects with GC_TENTATIVELY_UNREACHABLE still set are
candidates
    for collection.  If it's decided not to collect such an object
(e.g.,
    it has a __del__ method), its gc_refs is restored to GC_REACHABLE
again.



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