RFR: 8168914: Crash in ClassLoaderData/JNIHandleBlock::oops_do during concurrent marking

[Kim Barrett]

> On Feb 20, 2017, at 10:47 AM, Erik Helin <erik.helin at oracle.com> wrote:
> 
> Ok, lets see if we can wrap this up :)  I just uploaded a new version, 03:
> - incremental: http://cr.openjdk.java.net/~ehelin/8168914/02-03/
> - full: http://cr.openjdk.java.net/~ehelin/8168914/03/

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src/share/vm/classfile/classLoaderData.cpp 
 136   Chunk* volatile head = (Chunk*) OrderAccess::load_ptr_acquire((volatile intptr_t*)&_head);

The "volatile" in the declaration of head is unnecessary/misplaced.
It's not the variable "head" is modifiable by other threads.  Correct
would be "Chunk volatile * head", but then the volatile would need to
be const_casted away when initializing "c".  Better to just drop it,
as the load_acquire provides all the needed ordering.

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src/share/vm/classfile/classLoaderData.cpp 
 136   Chunk* volatile head = (Chunk*) OrderAccess::load_ptr_acquire((volatile intptr_t*)&_head);

Why the cast of &_head for the load_ptr_acquire?

Similarly, why the casts (both of them) here:
 127     OrderAccess::release_store_ptr((volatile intptr_t*) &_head, (intptr_t) next);

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