RFR: 8166811: Missing memory fences between memory allocation and refinement

[Thomas Schatzl]

Hi Kim,



On Thu, 2016-11-17 at 12:28 +0100, Thomas Schatzl wrote:
> Hi Kim,
> 
> 
[...]

> So the check in g1RemSet.cpp
> 
>  597   if (!r->is_old_or_humongous()) {
> 
> may filter the card out wrongly when processing the card from thread
> B
> as far as I can see.
> 
> That's why I remarked about only being able to filter out using
> is_young() here. For the refinement thread, "top" is current (after
> the
> fence), but the region type not (may still be "Free" until the
> refinement "synchronizes" with thread A in some way), doesn't it?
> 
> The change to "top" must have been observed already after the fence
> (in
> line 684) though and is safe to use (the allocation of the TLAB for
> thread B sets top using appropriate barriers, and the refinement will
> synchronize with whatever thread B set).
> 
> Probably I am overlooking something about how the type of region X
> set by thread A can be visible to refinement if it only
> "synchronizes" with thread B (that did not write the type of region
> X).

  I think it is good. Erik gave me the hint (and probably you already
mentioned it somewhere). That case can only happen for young regions,
and we can ignore them.

We only allocate into humongous regions once.

Thanks,
  Thomas