RFR (S): 8165859: gcTaskThread is missing volatile qualifier and barriers for _time_stamps

David Holmes david.holmes at oracle.com
Wed Sep 21 04:21:24 UTC 2016


On 21/09/2016 12:36 AM, Carsten Varming wrote:
> Dear Erik,
>
> See inline.
>
> On Tue, Sep 20, 2016 at 4:49 AM, Erik Österlund <erik.osterlund at oracle.com>
> wrote:
>>
>> I believe Kim is referring precisely to the release in the fence before
>> the compare-and-exchange in that comment you refer to, rather than the
>> membar Store* after the fence.
>>
>
> In this case there is no data, written prior to a successful exchange on
> _time_stamps, being passed to the reader of _time_stamps, so the fence
> before the exchange is not needed. In fact, it seems to me that a relaxed
> atomic cmpxchg (using memory_order_relaxed) is sufficient for the update
> and a plain read is sufficient to read _time_stamps.
>
> I would prefer to have explicit memory ordering between the linked producer
>> and consumer pair, as Kim suggests. This is how the JMM implements
>> sequential consistency - cooperatively with release_store_fence and
>> load_acquire pairs on the same data.
>>
>
> Hotspot is a C++ program. Mimicking the JMM in Hospot code is misguided
> (IMHO). In April, migrating the barriers towards the java model was
> suggested (
> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-April/019079.html),
> but the conclusion was a small move towards C++ 11 barriers (
> https://bugs.openjdk.java.net/browse/JDK-8155949). It would be great if
> there could be general consensus about the direction of changes to the
> memory barriers and their use in Hotspot (cc hotspot-runtime-dev as this
> was discussed there recently).

I don't see any connection between paired use of 
load-acquire/store-release and the JMM. ??

The paired use of release-store/load-acquire is where we are moving in 
the JVM - away from the cruder standalone barriers (storestore, 
storeload etc). But we can sometimes leverage the existence of 
safepoints to elide barriers.

We are looking at moving towards C++11 style APIs in the future for 
internal use - as per the recent cmpxchg update.

David
-----

>
>> It might very well be that specific architectures might work fine without
>> the acquire for the consumer as the producer implementation itself is
>> strong enough to guarantee sequential consistency, but I don't see why it
>> would be wise to bet on that if we don't have to, and it does not cost us
>> anything.
>>
>
> I don't undersand you reference to sequential consistency here as the only
> data being passed between threads is the value stored in the field updated
> by cmpxchg.
>
> Carsten
>
>
>> Thanks,
>> /Erik
>>
>>
>> On 2016-09-19 21:36, Carsten Varming wrote:
>>
>> Dear Kim,
>>
>> Reading a volatile field updated by cmpxchg does not need an acquire
>> operation.
>>
>> BTW. The specification in atomic.hpp says cmpxchg provides: "<fence>
>> compare-and-exchange <membar StoreLoad|StoreStore>". The release is part of
>> the fence. Are you suggesting that there is a release after the exchange?
>>
>> Carsten
>>
>> On Mon, Sep 19, 2016 at 3:18 PM, Kim Barrett <kim.barrett at oracle.com>
>> wrote:
>>
>>>> On Sep 19, 2016, at 2:26 PM, Carsten Varming <varming at gmail.com> wrote:
>>>>
>>>> Dear Erik,
>>>>
>>>> According to orderAccess.hpp an acquire is supposed to be paired with a
>>> release. It doesn't look like there is any synchronization on the data
>>> written to the time stamp array, so what exactly is going on?
>>>>
>>>> Carsten
>>>
>>> _time_stamps gets written with a cmpxchg_ptr (line 63 of
>>> gcTaskThread.cpp), which includes release semantics for the write; see
>>> atomic.hpp.
>>>
>>>
>>
>>


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