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Hi everyone,<br>
<br>
Could I have a couple of reviews for this change?<br>
<br>
<a class="moz-txt-link-freetext" href="https://bugs.openjdk.java.net/browse/JDK-8129626">https://bugs.openjdk.java.net/browse/JDK-8129626</a><br>
<a class="moz-txt-link-freetext" href="http://cr.openjdk.java.net/~brutisso/8129626/webrev.00/">http://cr.openjdk.java.net/~brutisso/8129626/webrev.00/</a><br>
<br>
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().<br>
<br>
Thanks to Per and Bertrand for helping out identifying and sorting
this out.<br>
<br>
From the bug report:<br>
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ConcurrentMarkThread::during_cycle() is implemented as:
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bool during_cycle() { return started() || in_progress(); }
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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():
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void ConcurrentMarkThread::sleepBeforeNextCycle() {
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// We join here because we don't want to do the
"shouldConcurrentMark()"
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// below while the world is otherwise stopped.
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assert(!in_progress(), "should have been cleared");
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MutexLockerEx x(CGC_lock, Mutex::_no_safepoint_check_flag);
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while (!started() && !_should_terminate) {
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CGC_lock->wait(Mutex::_no_safepoint_check_flag);
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}
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if (started()) {
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set_in_progress();
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clear_started();
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}
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}
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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. <br>
<br>
<br>
Thanks,<br>
Bengt<br>
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