RFR (XS): 8054808: Bitmap verification sometimes fails after Full GC aborts concurrent mark

[Thomas Schatzl]

Hi,

On Wed, 2014-08-27 at 08:00 -0700, Jon Masamitsu wrote:
> Thomas,
> 
> Would it work to snapshot the value for end()
> 
> end =r-> end()
> 
> and use that instead of calculating "end" from
> r->bottom()?

  not necessarily. As r->_end is not volatile the compiler is free to
replace any local variable with a re-read of it, resulting in the same
code and problem as before.

You would need something like "HeapWord* end = (HeapWord*
volatile)r->end();" to make sure that this replacement not done by the
compiler.

The proposed solution seemed simplest to me, since it avoids thinking
about what the contents of r->_end could be for the different types of
region.

E.g. the _end of humongous continues regions == _bottom iirc, so there
might be cases when we miss checking the bitmap. For example, due to
timing the code reads r->end() == r->bottom() + HeapRegion::GrainWords
for a region that has just been changed from Humongous Starts to normal,
but still gets the r->end() == r->bottom() for humongous continues
regions.

Above might not be possible for various reasons (I remember code at
least for creating humongous regions that makes sure that the various
_end values are written in a particular order), but to verify that I
would have to look in other places.

The current solution avoids requiring me to reason about this
completely.

Thanks,
  Thomas