8216987: ciMethodData::load_data() unpacks MDOs with non-atomic copy

[Erik Österlund]

Hi,

The ciMethodData::load_data() member function copies a raw MDO to the 
compiler mirror of said MDO. However, the copy is performed using a 
non-atomic copy function, despite being updated concurrently. This could 
potentially cause word tearing when reading metadata pointers, causing 
the VM to crash... in theory.

While this is not a problem when unpacking the extra data section, 
because it is done under a lock, the same can not be said about the rest 
of the MDO. So it should either be protected by a lock, or use an atomic 
copy function instead.

This patch adds an extra seat belt by performing atomic heap word copy 
instead.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8216987/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8216987

Thanks,
/Erik