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

[Erik Helin]

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,
Erik

> Testing:
> hs tier1+2; note that this has only been reported for the IBM xlc
> compiler for PPC which Oracle does not test for. I hope somebody at SAP
> can confirm that it generates the correct code now.
> 
> Based on JDK-8193063 also out for review right now.
> 
> Thanks,
>    Thomas
>