RFR (s): 8251460: Fix the biased-locking code in ObjectSynchronizer::FastHashCode
David Holmes
david.holmes at oracle.com
Wed Aug 12 04:23:40 UTC 2020
Bug: https://bugs.openjdk.java.net/browse/JDK-8251460
webrev: http://cr.openjdk.java.net/~dholmes/8251460/webrev/
During the review for JDK-8250606 it was noted that the biased-locking
section of FastHashCode contained a similar assertion regarding
execution at a safepoint:
// Relaxing assertion for bug 6320749.
assert(Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(),
"biases should not be seen by VM thread here");
which gives the impression that it is okay to execute this code block at
a safepoint if verify_in_progress() is true. But if we examine the next
line of code:
BiasedLocking::revoke(hobj, JavaThread::current());
we find that at a safepoint:
a) JavaThread::current() will assert because the current thread will be
the VMThread; and
b) BiasedLocking::revoke itself contains an assertion that it is not
invoked at a safepoint.
In practice there is no issue with revoking the bias at a safepoint, we
just have to use the right code to do so (as done elsewhere in the same
file):
if (SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::revoke_at_safepoint(hobj);
} else {
BiasedLocking::revoke(hobj, self);
}
Thanks,
David
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