An interesting bug

[Andrew Haley]

This is JDK8 on AArch64.

I have an interesting bug which causes a crash.  Occasionally, but
rarely, I get a crash when accessing an instance of ForkJoinTask.  The
instance is allocated in the old generation of the parallel scan GC.
However, the "this" pointer of the ForkJoinTask points into the middle
of another allocated object.

I'm trying to figure out how this might happen.  Perhaps the root scan
of the thread running the ForkJoinTask did not happen, or for some
reason the roots it found were not added to the root set.  Or perhaps
they were found, but were not rewritten in the thread.

Has anyone seen anything like this before?  Do you have any advice for
me?

Thanks,
Andrew.