Safepointing in GC

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index f154078809a9e83688f25035e2199f177a4074d9..285354c51501ca2d241204f9acc9678d771d75e1 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -16,6 +16,7 @@
 #include "vm/object_id_ring.h"
 #include "vm/stack_frame.h"
 #include "vm/store_buffer.h"
+#include "vm/thread_registry.h"
 #include "vm/verified_memory.h"
 #include "vm/verifier.h"
 #include "vm/visitor.h"
@@ -803,6 +804,9 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   ASSERT(!scavenging_);
   scavenging_ = true;
   Isolate* isolate = heap_->isolate();
+
+  isolate->thread_registry()->SafepointThreads();

[Ivan Posva, 2015/08/25 23:27:15]

ditto about the comment.

Also please explain what will happen if two different threads decide to race for
triggering a GC at the same time.

[koda, 2015/08/26 01:09:14]

Done.

Added TODO about avoiding the possibility of redundant collections by moving
safepointing into allocation failure/retry logic.

+
   PageSpace* page_space = heap_->old_space();
   NoSafepointScope no_safepoints;
 
@@ -858,6 +862,8 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
     OS::PrintErr(" done.\n");
   }
 
+  isolate->thread_registry()->ResumeAllThreads();
+
   // Done scavenging. Reset the marker.
   ASSERT(scavenging_);
   scavenging_ = false;