RFR(S): JDK-8139868: CMSScavengeBeforeRemark broken after JDK-8134953

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this fix for a GC id issue?

http://cr.openjdk.java.net/~brutisso/8139868/webrev.00/index.html
https://bugs.openjdk.java.net/browse/JDK-8139868

 From the CR:

Running with -XX:+CMSScavengeBeforeRemark will do an extra call to 
GenCollectedHeap::do_collection() inside 
CMSCollector::checkpointRootsFinal().

Inside CMSCollector::checkpointRootsFinal() we already have a valid GC 
id set up, but the call to GenCollectedHeap::do_collection() will create 
a new one using a GCIdMark, which means that when it returns the GC id 
is set to undefined.

It is correct that GenCollectedHeap::do_collection() creates a new GC id 
since we are doing one extra GC there. But it should not reset the GC id 
to undefined when it is done.


The fix is to use a GCIdMarkAndRestore inside 
GenCollectedHeap::do_collection() instead. This will preserve the 
"outer" GC id for the remark. But since the two other calls to 
GenCollectedHeap::do_collection() are for distinct GCs we can not assume 
that there always is an "outer" GC id to preserve. Thus, the constructor 
for GCIdMarkAndRestore has to use current_raw() instead of current() to 
store the previous GC id.

Thanks,
Bengt