RFR (s): 8251460: Fix the biased-locking code in ObjectSynchronizer::FastHashCode

[Patricio Chilano]

Hi David,

Changes look good to me.

Thanks,
Patricio
On 8/12/20 1:23 AM, David Holmes wrote:
> 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