RFR(S): 8206003: SafepointSynchronize with TLH: StoreStore barriers should be moved out of the loop

[David Holmes]

Hi Martin,

On 29/06/2018 8:12 PM, Doerr, Martin wrote:
> Thank you for the reviews.
> 
> I've created a new webrev with a "_release" version instead of "_no_release":
> http://cr.openjdk.java.net/~mdoerr/8206003_tlh_sync_membars/webrev.01/

That all seems fine.

> Due to this change, SafepointMechanism::initialize_header doesn't use a release barrier anymore which should be fine.

I agree. The JavaThread being constructed does not yet have a native 
thread associated with it so there is no "acquire" for a "release" to 
pair with in this case. Native thread creation/execution has its own 
memory barriers.

> Pushed to jdk/submit11 and our internal testing.

I'll put this through our internal testing too.

Thanks,
David

> 
> Best regards,
> Martin
> 
> 
> -----Original Message-----
> From: David Holmes [mailto:david.holmes at oracle.com]
> Sent: Freitag, 29. Juni 2018 00:49
> To: Erik Österlund <erik.osterlund at oracle.com>; Doerr, Martin <martin.doerr at sap.com>; hotspot-runtime-dev at openjdk.java.net; Robbin Ehn <robbin.ehn at oracle.com>; Andrew Haley (aph at redhat.com) <aph at redhat.com>
> Subject: Re: RFR(S): 8206003: SafepointSynchronize with TLH: StoreStore barriers should be moved out of the loop
> 
> On 29/06/2018 1:28 AM, Erik Österlund wrote:
>> Hi Martin,
>>
>> This did catch my eye too. This looks good to me. But could you consider
>> having _release in the name of the setter that uses release, and no
>> postfix for the one using a plain store, instead of giving that one a
>> _no_release postfix. I don't need another webrev.
> 
> +1
> 
> I'm assuming that nothing may be tripped up (ie assertion somewhere) if
> the polling status of different threads can now be seen out-of-order.
> 
> Thanks,
> David
> 
>>
>> Thanks,
>> /Erik
>>
>> On 2018-06-28 16:52, Doerr, Martin wrote:
>>> Hi,
>>>
>>> I have recently come across a bad placement of memory barriers in
>>> SafepointSynchronize::begin() and end() which were changed for JEP
>>> 312: Thread-Local Handshakes. They iterate over all JavaThreads and
>>> call SafepointMechanism::arm_local_poll or disarm_local_poll.
>>> Unfortunately, the release barriers are inside the latter functions.
>>>
>>> Assume we have several 1000 JavaThreads. This means the code executes
>>> several 1000 release barriers on weak memory model platforms (PPC64
>>> and ARM/aarch64). Only one is needed.
>>>
>>> A goal of JEP 312 was to minimize latency of safepoints which gets
>>> defeated by this issue to some extend on these platforms.
>>>
>>> It could be fixed by this proposal:
>>> http://cr.openjdk.java.net/~mdoerr/8206003_tlh_sync_membars/webrev.00/
>>>
>>> Please review.
>>>
>>> Best regards,
>>> Martin
>>>
>>