RFR: 8190711: Assert in G1MMUTracker due to concurrent modification

[Thomas Schatzl]

Hi,

On Wed, 2017-11-08 at 11:47 +0100, 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.

  looks good.

There does not seem to be any difference in the number of entries we
can have in that list, as add_pause() is the only one adding entries
anyway.
As you note we might only get some more expired entries to work
through.

Thanks,
  Thomas