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

[Zhengyu Gu]

On Thu, 11 Feb 2021 16:06:37 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahForwarding.inline.hpp line 89:
>> 
>>> 87:   // (would be paired with acquire in forwardee accessors). Acquire on failed update
>>> 88:   // would get the updated object after the forwardee load.
>>> 89:   markWord prev_mark = obj->cas_set_mark(new_mark, old_mark, memory_order_acq_rel);
>> 
>> Same here, CAS guarantees you to see latest mark word. memory_order_release should be sufficient here, paired with memory_order_acquire in resolve_forward barrier to ensure safe publishing the new object.
>
> Again, not really about the mark word. The transitive load of the object from that fwdptr is what we are after.

I kind of seeing what you meant, if there is a load after, we do need acquire. So good to me.

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