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

[Bengt Rutisson]

On 2015-10-20 14:19, Jesper Wilhelmsson wrote:
> Looks good!

Thanks, Jesper!

Bengt

> /Jesper
>
> Den 19/10/15 kl. 10:24, skrev 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