Assertion failure on PPC64 after 8200545: Improve filter for enqueued deferred cards

[Thomas Schatzl]

Hi Martin,

On Fri, 2019-05-24 at 16:54 +0200, Thomas Schatzl wrote:
> Hi,
> 
> On Fri, 2019-05-24 at 13:48 +0000, Doerr, Martin wrote:
> > Hi Thomas,
> > 
> > > The only way I could imagine such an error would be if the
> > > compiler did something weird with writing the fields of the heap
> > > attribute table? I.e. it loads a machine word (containing four of
> > > those RegionAttr entries), modifies just one of the bytes, and
> > > writes back the whole word. I.e. then some concurrent reader
> > > might see inconsistent values that flip back and forth.
> > > I really doubt this is the case though. Particularly I assume
> > > that at least on ppc64/linux you also use gcc.
> > 
> > 
[...]
> I can prepare some change for you to test, making the fields accessed
> "atomically" (via  e.g. our Atomic::load/store methods, or making
> them word sized) but I can't really test if it fixes the problem as
> we've never reproduced this case afaict in our CI. Would that help
> you?

Changes:
http://cr.openjdk.java.net/~tschatzl/8224681/webrev.test1/ uses
Atomic::load/store to access the G1HeapRegionAttr members only.

http://cr.openjdk.java.net/~tschatzl/8224681/webrev.test2/ test1 +
using 32 bit words for the members (64 bit struct). So above problem
should not happen.

I will follow up with some memory visibility analysis in the next days.

Thanks,
  Thomas