RFR: 8190711: Assert in G1MMUTracker due to concurrent modification

[Robbin Ehn]

Thanks for fixing this fast!

 From what I can tell, looks good!

/Robbin

On 11/08/2017 11:47 AM, Stefan Johansson wrote:
> Hi all,
> 
> Please review this fix for:
> https://bugs.openjdk.java.net/browse/JDK-8190711
> 
> Webrev:
> http://cr.openjdk.java.net/~sjohanss/8190711/00/
> 
> Summary:
> The G1MMUTracker is used both by the concurrent gc threads and during STW by the 
> VM thread. Up until this change we have had modifications done to the internal 
> data structure from both sides. To synchronize we have used the 
> SuspendibleThreadSet but this doesn't stop two different concurrent threads from 
> modifying the data at the same time. We need to prevent this.
> 
> The reason to update the data structure (remove entires) in the concurrent phase 
> is a possible performance improvement due to having fewer entries to iterate 
> when calling when_sec(). Since there can never be more than 64 entries, the 
> gain, if any, is very small. We still remove expired entries when adding new 
> ones, done by add_pause() during STW. To avoid having more than one thread doing 
> modification of the list this change removes the call to 
> remove_expired_entries() in when_sec(). Having the SuspendibleThreadSet will 
> ensure the the concurrent threads won't access the data while the VM thread is 
> updating it.
> 
> Testing:
> * Tier 1-2
> 
> Thanks,
> Stefan