Add a read barrier for weak roots in monitors.

A weak root requires a read barrier for the to-space invariant to hold
because the object pointed to by a weak root can't be marked/forwarded
like the one pointed to by a strong root (GC does not know if it's
alive or not at that point) and because, without a read barrier, a
mutator could access it and obtain a from-space reference, which would
violate the to-space invariant.

TODO: do similar for the other types of weak roots.

Bug: 12687968
Change-Id: I563a0fa4f875e0c21ac96f57696959454e13b15a

1 files changed, 6 insertions, 2 deletions

diff --git a/runtime/monitor.h b/runtime/monitor.h
index bc5d2e4..ed1b27b 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -27,6 +27,7 @@
 #include "atomic.h"
 #include "base/mutex.h"
 #include "object_callbacks.h"
+#include "read_barrier.h"
 #include "thread_state.h"
 
 namespace art {
@@ -92,8 +93,9 @@ class Monitor {
 
   static bool IsValidLockWord(LockWord lock_word);
 
+  template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   mirror::Object* GetObject() const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    return obj_;
+    return ReadBarrier::BarrierForWeakRoot<mirror::Object, kReadBarrierOption>(obj_);
   }
 
   void SetObject(mirror::Object* object);
@@ -190,7 +192,9 @@ class Monitor {
   // Owner's recursive lock depth.
   int lock_count_ GUARDED_BY(monitor_lock_);
 
-  // What object are we part of.
+  // What object are we part of. This is a weak root. Do not access
+  // this directly, use GetObject() to read it so it will be guarded
+  // by a read barrier.
   mirror::Object* obj_;
 
   // Threads currently waiting on this monitor.