Fix a race condition in RosAlloc.

Fix a race condition in RosAlloc between RosAlloc::BulkFree() and
RosAlloc::RevokeThreadLocalRuns() with regard to bulk_free_bit_map.

Change-Id: I128917d5bdfe2dab604174ca4cbe228282578b8a
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>
Bug: 12592026
diff --git a/runtime/gc/allocator/rosalloc.h b/runtime/gc/allocator/rosalloc.h
index 4eb13315..7480975 100644
--- a/runtime/gc/allocator/rosalloc.h
+++ b/runtime/gc/allocator/rosalloc.h
@@ -456,7 +456,9 @@
   // and the footprint.
   Mutex lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
   // The reader-writer lock to allow one bulk free at a time while
-  // allowing multiple individual frees at the same time.
+  // allowing multiple individual frees at the same time. Also, this
+  // is used to avoid race conditions between BulkFree() and
+  // RevokeThreadLocalRuns() on the bulk free bitmaps.
   ReaderWriterMutex bulk_free_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
 
   // The page release mode.