Merge "Document and use AtomicStack concurrency properties"

1 files changed, 8 insertions, 1 deletions

diff --git a/runtime/gc/accounting/atomic_stack.h b/runtime/gc/accounting/atomic_stack.h
index 351798efd1..3d0e8172b6 100644
--- a/runtime/gc/accounting/atomic_stack.h
+++ b/runtime/gc/accounting/atomic_stack.h
@@ -29,6 +29,13 @@
 #include "mem_map.h"
 #include "stack_reference.h"
 
+// This implements a double-ended queue (deque) with various flavors of PushBack operations,
+// as well as PopBack and PopFront operations. We expect that all calls are performed
+// by a single thread (normally the GC). There is one exception, which accounts for the
+// name:
+// - Multiple calls to AtomicPushBack*() and AtomicBumpBack() may be made concurrently,
+// provided no other calls are made at the same time.
+
 namespace art {
 namespace gc {
 namespace accounting {
@@ -150,7 +157,7 @@ class AtomicStack {
   // Pop a number of elements.
   void PopBackCount(int32_t n) {
     DCHECK_GE(Size(), static_cast<size_t>(n));
-    back_index_.FetchAndSubSequentiallyConsistent(n);
+    back_index_.StoreRelaxed(back_index_.LoadRelaxed() - n);
   }
 
   bool IsEmpty() const {