RFR: 8166862: CMS needs klass_or_null_acquire

[Mikael Gerdin]

Hi,

On 2016-10-03 11:25, Thomas Schatzl wrote:
> 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.

Looks good to me as well.
/Mikael

>
> Thomas
>