[10?] RFR (XS): 8129440: G1 crash during concurrent root region scan

[Thomas Schatzl]

Hi,

On Thu, 2017-12-07 at 10:14 +0100, Erik Helin wrote:
> On 12/06/2017 08:17 PM, Thomas Schatzl wrote:
> > Hi all,
> > 
> >    can I have reviews for this small change that prevents C++
> > compilers from generating code that reloads oop value from the heap
> > instead of using a local variable in concurrent marking closures.
> > This can cause issues if this reloading occurs after the originally
> > loaded value has been checked for NULL to prevent SIGSEGVs and the
> > mutator changed the value in the meantime.
> > 
> > E.g.
> > 
> >    oop o = load_from_heap(p);
> >    if (o == NULL) {
> >       return;
> >    }
> >    // do something with o; potential crash here when reloading from
> > p
> > instead of actually reusing o
> > 
> > According to SAP particularly the IBM xlc compiler tends to to such
> > nasty things (which is completely okay from a language POV).
> > 
> > The fix is to make the load from the heap a volatile load using the
> > new AccessBarrier API in cases this can happen.
> > 
> > CR:
> > https://bugs.openjdk.java.net/browse/JDK-8129440
> > Webrev:
> > http://cr.openjdk.java.net/~tschatzl/8129440/webrev/
> 
> Looks good, Reviewed.
> 

  thanks for your review.

Thomas