RFR 8240918: [REDO] Allow direct handshakes without VMThread intervention

David Holmes david.holmes at oracle.com
Fri Apr 3 13:21:30 UTC 2020

Hi Patricio,

On 3/04/2020 6:00 pm, Patricio Chilano wrote:
> Hi Martin,
> On 4/2/20 5:24 PM, Doerr, Martin wrote:
>> Hi Patricio,
>> right, you got my point. Sorry for not explaining it in greater detail 
>> at the beginning. I was busy with other things.
>> There should be a memory barrier ordering read of _pending_threads 
>> (which is observed as 0) and whatever gets done when returning from 
>> execute_direct.
>> I don't like relying on barriers which are hidden somewhere and may be 
>> subject to change.
> Yes, I agree with that. I added an explicit acquire fence at the end of 
> Handshake::execute_direct() with a comment. Thanks for noticing it.

This is unclear to me:

281   // Prevent future loads from floating above the load of 
  282   // in is_completed(). This prevents reading stale data modified 
in the
  283   // handshake closure by the Handshakee.
  284   OrderAccess::acquire();
  286   return op.executed();

An acquire() by itself has no meaning, it has to be associated with a 
release() and both have to be associated with the load, respectively, 
store, of the same variable. From the comment it sounds like you just 
want a loadload() barrier here.

Other updates seem fine.


>> I'd also prefer explicit barriers on the writers side over the 
>> implicit ones from Atomic::add, because explicit barriers with 
>> comments help a lot understanding the code.
>> (Not sure if you want to change that. Just my opinion.)
> I added a comment on top of Atomic::dec() on why we need a memory 
> barrier there and the fact that we are already using a full memory fence.
> If you still think I should add an explicit barrier for code readability 
> I can add it though.
> Here is v3 which also addresses Dan's comments.
> Full:
> http://cr.openjdk.java.net/~pchilanomate/8240918/v3/webrev/
> Inc:
> http://cr.openjdk.java.net/~pchilanomate/8240918/v3/inc/webrev/
> Thanks!
> Patricio
>> Best regards,
>> Martin
>>> -----Original Message-----
>>> From: Patricio Chilano <patricio.chilano.mateo at oracle.com>
>>> Sent: Donnerstag, 2. April 2020 19:56
>>> To: Robbin Ehn <robbin.ehn at oracle.com>; Doerr, Martin
>>> <martin.doerr at sap.com>; hotspot-runtime-dev at openjdk.java.net
>>> Subject: Re: RFR 8240918: [REDO] Allow direct handshakes without
>>> VMThread intervention
>>> On 4/2/20 1:16 PM, Patricio Chilano wrote:
>>>> Hi Martin,
>>>> Thanks for looking at this.
>>>> On 4/2/20 10:15 AM, Robbin Ehn wrote:
>>>>> Hi Martin,
>>>>> On 2020-04-02 14:55, Doerr, Martin wrote:
>>>>>> Hi all,
>>>>>> first of all, thanks, Patricio, for doing this.
>>>>>> One question:
>>>>>> _pending_threads is used for synchronizing threads.
>>>>>> Don't we need release / acquire semantics?
>>>>>> I guess otherwise handshaker can read stale data from handshakee
>>>>>> after observing is_completed.
>>>>>> Or did I miss anything?
>>>>> Reading stale should be fine in this case, since the counter goes
>>>>> from n->0, thus you can only get false negatives from "bool
>>>>> is_completed()" ?
>>>> Right, as Robbin points out at most we will just have to loop again.
>>>> Also, when the handshakee processes the handshake, right after
>>>> updating _pending_threads, it will either signal the _processing_sem
>>>> if there are no direct operations, or it will signal the
>>>> _handshake_turn_sem, so that will provide the release already. The
>>>> next read in the loop should read the updated value. Are you okay with
>>>> that?
>>> I realize you might be thinking of a different case now. You mean the
>>> handshaker could execute loads from code after
>>> Handshake::execute_direct() while it hasn't yet read _pending_threads
>>> from is_completed()? Although there is a load_acquire() in
>>> ~ThreadsListHandle() that will prevent those issues you might argue that
>>> we can't rely on code which might change.
>>> I think (not counting that load_acquire from ~ThreadsListHandle) the
>>> issue could appear for example if you created a ThreadsListHandle first,
>>> then called Handshake::execute_direct(), and when returning you try to
>>> read some field from the handshakee (you might want to assert something
>>> for example). If there wasn't any ThreadsListHandle created before
>>> calling execute_direct() I think you are not allowed to access other
>>> JavaThread's data anyway since the thread might have exited already,
>>> unless you create a ThreadsListHandle which will already provide the
>>> acquire. So I think you are right and we might need a fence before
>>> returning from execute_direct() or add a comment that we are relying on
>>> the load_acquire from ~ThreadsListHandle(). Is that what you were 
>>> thinking?
>>> Thanks,
>>> Patricio
>>>> Thanks!
>>>> Patricio
>>>>> Thanks, Robbin
>>>>>> Best regards,
>>>>>> Martin
>>>>>>> -----Original Message-----
>>>>>>> From: hotspot-runtime-dev <hotspot-runtime-dev-
>>>>>>> bounces at openjdk.java.net> On Behalf Of Robbin Ehn
>>>>>>> Sent: Donnerstag, 2. April 2020 13:36
>>>>>>> To: Patricio Chilano <patricio.chilano.mateo at oracle.com>;
>>>>>>> hotspot-runtime-
>>>>>>> dev at openjdk.java.net
>>>>>>> Subject: Re: RFR 8240918: [REDO] Allow direct handshakes without
>>>>>>> VMThread intervention
>>>>>>> Hi Patricio,
>>>>>>> Thanks for fixing, looks good.
>>>>>>> /Robbin
>>>>>>> On 2020-04-01 19:45, Patricio Chilano wrote:
>>>>>>>> Hi all,
>>>>>>>> Please review this redo for 8230594. Since the reason behind the
>>>>>>>> Windows
>>>>>>>> timeouts turned out to be 8240902, this patch is almost identical
>>>>>>>> to the
>>>>>>>> reverse of the backout modulo the rebase.
>>>>>>>> There is only one small fix for an issue not identified in the
>>>>>>>> original
>>>>>>>> patch which I explained in the comments. To avoid this possible
>>>>>>>> issue of
>>>>>>>> deleting a JavaThread that called Thread::destroy_vm() while there
>>>>>>>> are
>>>>>>>> handshakers blocked in the handshake_turn_sem semaphore, I
>>> added a
>>>>>>> check
>>>>>>>> to verify the JavaThread in question is not protected by some
>>>>>>>> ThreadsList reference before attempting the delete it. In case 
>>>>>>>> it is
>>>>>>>> protected, we need to at least clear the value for _thread_key to
>>>>>>>> avoid
>>>>>>>> possible infinite loops at thread exit (this can happen in 
>>>>>>>> Solaris).
>>>>>>>> The only other change is that I removed the check for local 
>>>>>>>> polls in
>>>>>>>> Handshake::execute_direct() since after the backout, thread-local
>>>>>>>> handshakes was implemented for arm32 which was the only
>>> remaining
>>>>>>> platform.
>>>>>>>> I tested it several times in mach5 tiers 1-6 and once in t7 and
>>>>>>>> saw no
>>>>>>>> failures.
>>>>>>>> Bug:
>>>>>>>>       https://bugs.openjdk.java.net/browse/JDK-8240918
>>>>>>>> Webrev:
>>>>>>>> http://cr.openjdk.java.net/~pchilanomate/8240918/v1/webrev/
>>>>>>>> Thanks,
>>>>>>>> Patricio

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