(S) RFR: 8157904: Atomic::cmpxchg for jbyte is missing a fence on initial failure

[David Holmes]

On 24/08/2016 5:36 PM, Andrew Haley wrote:
> On 22/08/16 08:54, David Holmes wrote:
>> An earlier code review noticed that the default shared implementation of
>> Atomic::cmpxchg(jbyte*) was missing the required post-memory-barrier in
>> case of an initial failure:
>
> Just to satisfy my curiosity, why is the post-memory-barrier in
> case of an initial failure required?  Is there some specification
> somewhere that I can refer to?

In atomic.hpp:

   // All of the atomic operations that imply a read-modify-write action
   // guarantee a two-way memory barrier across that operation.

and then each op has additional descriptions e.g.:

   // Performs atomic compare of *dest and compare_value, and exchanges
   // *dest with exchange_value if the comparison succeeded. Returns prior
   // value of *dest. cmpxchg*() provide:
   // <fence> compare-and-exchange <membar StoreLoad|StoreStore>

David

> Thanks,
>
> Andrew.
>