RFR: JDK-8248725: Shenandoah: Claim verifier thread roots for parallel processing

[Zhengyu Gu]

On 7/2/20 5:03 PM, rkennke at redhat.com wrote:
> We see occasional crashes during CI testing:
> 
> # Internal Error (/home/jenkins/workspace/nightly/jdk-
> jdk/src/hotspot/cpu/x86/frame_x86.cpp:684), pid=25982, tid=26009
> # assert(_last_Java_pc == NULL) failed: already walkable
> 
> This seems to be caused by calls to
> Threads::possibly_parallel_oops_do() with par=false even though it's
> called by multiple workers. This leads to threads being visited by more
> than one worker, and thus trip this assert.

You need to change thread claim token for parallel iteration in root 
verifier constructor.

-Zhengyu

> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8248725
> Webrev:
> http://cr.openjdk.java.net/~rkennke/JDK-8248725/
> 
> Testing: multiple runs of hotspot_gc_shenandoah (which exposed the bug
> before)
> 
> Is the right way to push this to: http://hg.openjdk.java.net/jdk/jdk15/
>   ?
> 
> Roman
>