Safepointing in GC

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index f45c5b04a95f5715b7f77d3ab3e2ccd2571e8ffe..79ea761468f0af3acdf39517e0b8662c296803c7 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -11,6 +11,7 @@
 #include "vm/lockers.h"
 #include "vm/object.h"
 #include "vm/os_thread.h"
+#include "vm/thread_registry.h"
 #include "vm/verified_memory.h"
 #include "vm/virtual_memory.h"
 
@@ -767,6 +768,12 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks) {
     }
     set_tasks(1);
   }
+  // Ensure that all threads for this isolate are at a safepoint (either stopped
+  // or in native code). If two threads are racing at this point, the loser
+  // will continue with its collection after waiting for the winner to complete.
+  // TODO(koda): Consider moving SafepointThreads into allocation failure/retry
+  // logic to avoid needless collections.
+  isolate->thread_registry()->SafepointThreads();
 
   // Perform various cleanup that relies on no tasks interfering.
   isolate->class_table()->FreeOldTables();
@@ -907,6 +914,8 @@ void PageSpace::MarkSweep(bool invoke_api_callbacks) {
     freelist_[HeapPage::kExecutable].Print();
   }
 
+  isolate->thread_registry()->ResumeAllThreads();
+
   // Done, reset the task count.
   {
     MonitorLocker ml(tasks_lock());