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

[Thomas Schatzl]

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