RFR: 8261492: Shenandoah: reconsider forwardee accesses memory ordering [v2]

[Zhengyu Gu]

On Thu, 11 Feb 2021 16:04:38 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahForwarding.inline.hpp line 58:
>> 
>>> 56:   assert(Thread::current()->is_Java_thread(), "Must be a mutator thread");
>>> 57: 
>>> 58:   markWord mark = obj->mark_acquire();
>> 
>> Actually, I would argue that you don't need acquire here, since you don't touch anything other than mark word, so that there is no order that needs to be enforced.
>
> It is not about the mark word itself, we access the forwardee afterwards. It is about transitive dependency: object copy stores -> forwardee installation (markword store, release) -> forwardee discovery (markword load, acquire) --> object copy access. Remember this: https://github.com/openjdk/jdk/pull/2498#discussion_r574498830

Sorry. I commented on wrong place. I actually meant line #78 in try_update_forwardee().

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PR: https://git.openjdk.java.net/jdk/pull/2496