RFR (S): 8200385: Eagerly reclaimed humongous objects leave mark in prev bitmap

[Thomas Schatzl]

Hi,

  could a second person look at this please?

Thanks,
  Thomas

On Thu, 2018-03-29 at 11:12 +0200, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for this small fix to a benign bug, that is I
> haven't seen any actual product failure from it but some very rare
> test
> failures, where when we eagerly reclaim humongous objects we leave a
> mark on the prev bitmap in some cases?
> 
> The suggested fix is to always look at the prev bitmap and clear it,
> and if needed also clear potential marks in the next bitmap.
> 
> To make the failure appear basically 100% in that test, I added a
> simple assert after reclaiming the humongous object.
> 
> With the fix, this failure goes away completely.
> 
> Note that this is more a "data structure hygiene" fix - the stray
> mark
> on the prev bitmap will be automatically cleared after switching the
> bitmaps at cleanup and preparing that bitmap for the next mark.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8200385
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8200385/webrev/
> Testing:
> local testing with existing tests
> (gc/g1/TestEagerReclaimHumongousRegionsClearMarkBits.java)
> 
> Thanks,
>   Thomas