RFR(S): 8229422: Taskqueue: Outdated selection of weak memory model platforms

[Andrew Haley]

On 1/15/20 1:00 AM, David Holmes wrote:
> On 15/01/2020 2:15 am, Andrew Haley wrote:
>> On 1/14/20 3:52 PM, Doerr, Martin wrote:
>>
>>> good catch. I think you're right. A multi-copy-atomic, but weak
>>> architecture (e.g. aarch64) needs an instruction which orders both
>>> volatile loads.
>>
>> Good, I thought so.
>>
>> Given that TSO machines define OrderAccess::acquire() as no more than
>> a compiler barrier, I believe that we could do something like
>>
>> #ifdef CPU_MULTI_COPY_ATOMIC
>>     OrderAccess::acquire();
>> #else
>>     OrderAccess::fence();
>> #endif
>
> "acquire" isn't used to order loads it is used to pair with a "release"
> associated with the store of the variable now being loaded.
>
> If this is the code referred to:
>
>    Age oldAge = _age.get();
>    // Architectures with weak memory model require a barrier here
>    // to guarantee that bottom is not older than age,
>    // which is crucial for the correctness of the algorithm.
> #ifndef CPU_MULTI_COPY_ATOMIC
>    OrderAccess::fence();
> #endif
>    uint localBot = Atomic::load_acquire(&_bottom);
>
> then I think there is an assumption (perhaps incorrect) that the
> load_acquire will prevent reordering as well as performing the necessary
> "acquire" semantics.

It depends on how _age is written to.

As far as I can see there is no ordering between setting _bottom and setting
_age,

  void set_empty() {
    _bottom = 0;
    _age.set(0);
  }

so it looks like any kind of fence on the reader side is pointless anyway. In
that case, I don't know why we're doing any of this if it doesn't matter
what order the reader threads see updates to _age and _bottom.

It's all rather baffling. _bottom is declared volatile, as is _age, so I
guess there must be some ordering requirements, but no fences on the
writing side to enforce it.

What actually are the ordering requirements between _bottom and _age?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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