RFR: 8166862: CMS needs klass_or_null_acquire

[Thomas Schatzl]

Hi,

On Sat, 2016-10-01 at 17:29 -0400, Kim Barrett wrote:
> Please review this change to the CMS collector to replace uses of
> klass_or_null with klass_or_null_acquire where needed.
> 
> [Reviewing on hotspot-dev; only GC code is involved, but some non-GC
> folks have expressed interest.]
> 
> All non-assert uses of klass_or_null are being replaced with the
> acquire variant.  In most cases, the size is being accessed soon
> after, and the acquire is needed to ensure order of klass and
> possible inline size access.  In a few cases, the need for acquire is
> less immediately obvious:
> 
> - CompatibleFreeListSpace::block_is_obj - Size access is not
> lexically apparent, but callers access the size or other data ordered
> with the klass.
> 
> - MarkFromRootsClosure::do_bit and ParMarkFromRootsClosure::do_bit:
>   These perform other operations that need to be properly ordered
> wrto the klass access.
> 
> Removed a couple of explict acquire fences that are made redundant by
> the use of klass_or_null_acquire.  The latter is preferable, as it
> permits the use of platform-specific load_acquire implementations
> that might be better than separate load + acquire fence.
> 
> Removed a stale comment about permgen and is_parsable.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8166862
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8166862/webrev.00/

  looks good.

Thomas