RFR(S) 8067331: Zero: Atomic::xchg and Atomic::xchg_ptr need full memory barrier

[Coleen Phillimore]

This looks good.  Sorry for the delay reviewing it as it looks 
important.  Is there a small test case that can be written to test 
this?  (or no?)

Coleen

On 1/6/15, 8:47 PM, David Holmes wrote:
> Can we get a second hotspot review please - this is quite straight 
> forward.
>
> Thanks,
> David
>
> On 5/01/2015 12:21 PM, David Holmes wrote:
>> Hi Severin,
>>
>> Sorry this slipped through the cracks before the holidays.
>>
>> On 16/12/2014 1:09 AM, Severin Gehwolf wrote:
>>> Hi,
>>>
>>> Could someone please review and sponsor the following change?
>>>
>>> bug: https://bugs.openjdk.java.net/browse/JDK-8067331
>>> webrev:
>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8067331/webrev.02/
>>>
>>> The issue is that all atomic operations are expected to be full memory
>>> barriers, but two implementations of the Zero variant JVM - 
>>> Atomic::xchg
>>> and Atomic::xchg_ptr - use GCC's built-in __sync_lock_test_and_set 
>>> which
>>> is an acquire barrier only. The fix adds full memory barriers manually
>>> for those.
>>>
>>> Thoughts?
>>
>> Looks completely consistent with what I just saw in the aarch64 port 
>> code.
>>
>> I can sponsor this but we still need a second review.
>>
>> Thanks,
>> David
>>
>>> Cheers,
>>> Severin
>>>