RFR: JDK-8129626: G1: set_in_progress() and clear_started() needs a barrier on non-TSO platforms

[Bengt Rutisson]

Hi Bill,

On 2015-06-24 16:58, bill pittore wrote:
> Hi Bengt,
>  This seems to eliminate the TSO issues rather cleanly. 

Thanks for looking at this!

> One small nit is that the comments in concurrentMarkThread.hpp and 
> CollectedHeap.cpp talk about setting/clearing 'flags' whereas now it's 
> setting a state variable.

Agreed. I've updated the comment this way:

diff --git a/src/share/vm/gc/g1/concurrentMarkThread.hpp 
b/src/share/vm/gc/g1/concurrentMarkThread.hpp
--- a/src/share/vm/gc/g1/concurrentMarkThread.hpp
+++ b/src/share/vm/gc/g1/concurrentMarkThread.hpp
@@ -81,11 +81,11 @@
    void set_in_progress()   { assert(_state == Started, "must be 
starting a cycle"); _state = InProgress; }
    bool in_progress()       { return _state == InProgress; }

-  // This flag returns true from the moment a marking cycle is
+  // Returns true from the moment a marking cycle is
    // initiated (during the initial-mark pause when started() is set)
    // to the moment when the cycle completes (just after the next
    // marking bitmap has been cleared and in_progress() is
-  // cleared). While this flag is true we will not start another cycle
+  // cleared). While during_cycle() is true we will not start another cycle
    // so that cycles do not overlap. We cannot use just in_progress()
    // as the CM thread might take some time to wake up before noticing
    // that started() is set and set in_progress().


I'll push my change with this this updated comment.

Thanks again for reviewing this!

Bengt


>
> bill
>
> On 6/24/2015 3:28 AM, 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/
>>
>> 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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