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

[David Holmes]

Thanks Patricio!

David

On 13/08/2020 4:42 am, Patricio Chilano wrote:
> 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
>