[pypy-issue] Issue #2212: black -> white pointer in the GC (pypy/pypy)

[Armin Rigo]

New issue 2212: black -> white pointer in the GC
https://bitbucket.org/pypy/pypy/issues/2212/black-white-pointer-in-the-gc

Armin Rigo:

It is possible to hit this assertion in our incremental GC:

```
#!python

ll_assert(False, "black -> white pointer found")
```

because of the JIT's rewrite.py, which will not put a write barrier before a ``setfield(p0, p1)`` if ``p1`` is an old constant.  But p1 might be white and p0 black.  I've no clue why the problem doesn't show up more often, but it triggers the assert above with the proper `PYPY_GC_*` env vars.

I think that it is not an important issue: in this case, the constant `p1` is also always reachable from the CompiledLoop itself.  If we reach the assert in the GC, it's because the ``setfield`` was executed recently, during the same major collection iteration.  The loop itself is alive during this iteration---running JIT assembler relies on the loop being alive.  So we will anyway see the same ConstPtr from the loop.

Is the above reasoning correct?  Am I missing an edge case?