Scan dirty cards separate from aging them

Currently we scan objects corresponding to dirty cards while atomically
aging them. This could lead to following situation:

Mutator			GC-thread
-------			---------
1) dirty card
			a) age card
			b) scan ref
2) store reference

Although the write-barrier first stores the reference and then dirties
the corresponding card. However, it may reorder and lead to the above
situation.

This CL separates aging the cards and scanning them for dirty objects.
These two steps are done such that the mutator stack-scanning checkpoint
is between the two operations, ensuring that any in-progress write
barrier when cards were aged is finished before scanning the cards.

Bug: 288315519
Test: atest --rerun-until-failure 100 art-run-test-658-fp-read-barrier
Change-Id: Ia6f1c01905ba950047be528e3905809fe7a62df9

0 files changed, 0 insertions, 0 deletions