RFR: JDK-8129626: G1: set_in_progress() and clear_started() needs a barrier on non-TSO platforms
Bengt Rutisson
bengt.rutisson at oracle.com
Wed Jun 24 08:31:39 UTC 2015
On 2015-06-24 10:34, Per Liden wrote:
> On 2015-06-24 10:15, Bengt Rutisson wrote:
>>
>> Hi Per,
>>
>> Thanks for looking at this!
>>
>> On 2015-06-24 10:09, Per Liden wrote:
>>> Hi Bengt,
>>>
>>> On 2015-06-24 09:28, Bengt Rutisson wrote:
>>>>
>>>>
>>>> Hi Vitaly,
>>>>
>>>> On 2015-06-23 23:53, Vitaly Davidovich wrote:
>>>>>
>>>>> Naive question - could this be converted into a numeric state field
>>>>> that indicates the lifecycle (e.g 1 = in progress, 2 = started)?
>>>>>
>>>>
>>>> Good question! That's a much simpler and more stable solution.
>>>>
>>>> New webrev:
>>>> http://cr.openjdk.java.net/~brutisso/8129626/webrev.02/
>>>
>>> This looks much nicer, and I can't think of any reason why that
>>> wouldn't work. One little request though, can we name the states to
>>> match the function names, like Idle/Started/InProgress? And
>>> clear_in_progress() should probably be set_idle() to align with the
>>> rest.
>>
>> Yes, I was struggling a bit with the naming. What do you think about
>> this?
>>
>> cr.openjdk.java.net/~brutisso/8129626/webrev.03/
>
> Looks good!
Thanks Per!
Bengt
>
> cheers,
> /Per
>
>>
>> Thanks,
>> Bengt
>>>
>>> cheers,
>>> /Per
>>>
>>>>
>>>> Thanks,
>>>> Bengt
>>>>
>>>>
>>>>
>>>>> sent from my phone
>>>>>
>>>>> On Jun 23, 2015 5:42 PM, "bill pittore" <bill.pittore at oracle.com
>>>>> <mailto:bill.pittore at oracle.com>> wrote:
>>>>>
>>>>> Generally when you have a storestore on the write side you need a
>>>>> loadload on the read side to prevent the second read from
>>>>> floating
>>>>> above the first one. The reading thread could read in_progress as
>>>>> 0 before it reads starting. Meanwhile the write thread writes
>>>>> 1 to
>>>>> in_progress, issues storestore, clears starting. Reading thread
>>>>> then reads starting as 0. I don't know if the CGC mutex somehow
>>>>> eliminates this issue as I'm not familiar with the code in
>>>>> detail.
>>>>>
>>>>> bill
>>>>>
>>>>> On 6/23/2015 4:25 PM, Bengt Rutisson wrote:
>>>>>>
>>>>>> Hi everyone,
>>>>>>
>>>>>> Could I have a couple of reviews for this change?
>>>>>>
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8129626
>>>>>> http://cr.openjdk.java.net/~brutisso/8129626/webrev.00/
>>>>>> <http://cr.openjdk.java.net/%7Ebrutisso/8129626/webrev.00/>
>>>>>>
>>>>>> We need to add a barrier between the calls to set_in_progress()
>>>>>> and clear_started() to make sure that other threads sees the
>>>>>> correct value when they use
>>>>>> ConcurrentMarkThread::during_cycle().
>>>>>>
>>>>>> Thanks to Per and Bertrand for helping out identifying and
>>>>>> sorting this out.
>>>>>>
>>>>>> From the bug report:
>>>>>>
>>>>>> ConcurrentMarkThread::during_cycle() is implemented as:
>>>>>>
>>>>>> bool during_cycle() { return started() || in_progress(); }
>>>>>>
>>>>>> So, it checks both ConcurrentMarkThread::_started and
>>>>>> ConcurrentMarkThread::_in_progress and they are meant to
>>>>>> overlap.
>>>>>> That is, we should not set _started to false until after we have
>>>>>> set _in_progress to true. This is done in
>>>>>> sleepBeforeNextCycle():
>>>>>>
>>>>>> void ConcurrentMarkThread::sleepBeforeNextCycle() {
>>>>>> // We join here because we don't want to do the
>>>>>> "shouldConcurrentMark()"
>>>>>> // below while the world is otherwise stopped.
>>>>>> assert(!in_progress(), "should have been cleared");
>>>>>>
>>>>>> MutexLockerEx x(CGC_lock, Mutex::_no_safepoint_check_flag);
>>>>>> while (!started() && !_should_terminate) {
>>>>>> CGC_lock->wait(Mutex::_no_safepoint_check_flag);
>>>>>> }
>>>>>>
>>>>>> if (started()) {
>>>>>> set_in_progress();
>>>>>> clear_started();
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> On non-TSO platforms there is a risk that the write to
>>>>>> _in_progress (from set_in_progress()) is seen by other threads
>>>>>> after the write to _started (in clear_started()). In that case
>>>>>> there is a window when during_cycle() may return false even
>>>>>> though we are in a concurrent cycle.
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Bengt
>>>>>
>>>>
>>
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