Safepointing in GC

runtime/vm/thread_registry.h

Index: runtime/vm/thread_registry.h
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index efe81bdcfa69ca8540760424d8a289bca8943275..c758a89e7f04ca67f8616f3cde720a24d229f9a8 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -27,12 +27,16 @@ class ThreadRegistry {
   // Bring all threads in this isolate to a safepoint. The caller is
   // expected to be implicitly at a safepoint. The threads will wait
   // until ResumeAllThreads is called. First participates in any
-  // already pending rendezvous requested by another thread. Any
+  // already pending rendezvous requested by another thread.
+  //
+  // Returns an identifier of this rendezvous, which can be passed to
+  // Thread::EnterIsolateAsHelper to allow the thread free passage in/out
+  // of the isolate while this rendezvous is in progress. Otherwise, any
   // thread that tries to enter this isolate during rendezvous will
   // wait in RestoreStateTo. Nesting is not supported: the caller must
   // call ResumeAllThreads before making further calls to
   // SafepointThreads.
-  void SafepointThreads();
+  SafepointId SafepointThreads();
 
   // Unblocks all threads participating in the rendezvous that was organized
   // by a prior call to SafepointThreads.
@@ -49,7 +53,7 @@ class ThreadRegistry {
   bool RestoreStateTo(Thread* thread, Thread::State* state) {
     MonitorLocker ml(monitor_);
     // Wait for any rendezvous in progress.
-    while (in_rendezvous_) {
+    while (IsParticipant(thread)) {
       ml.Wait(Monitor::kNoTimeout);
     }
     Entry* entry = FindEntry(thread);
@@ -88,8 +92,8 @@ class ThreadRegistry {
     ASSERT(entry->scheduled);
     entry->scheduled = false;
     entry->state = state;
-    if (in_rendezvous_) {
-      // Don't wait for this thread.
+    if (IsParticipant(thread)) {
+      // Update count (don't wait for this thread when resuming).
       ASSERT(remaining_ > 0);
       if (--remaining_ == 0) {
         ml.NotifyAll();
@@ -158,9 +162,11 @@ class ThreadRegistry {
   // Note: Lock should be taken before this function is called.
   void CheckSafepointLocked();
 
-  // Returns the number threads that are scheduled on this isolate.
-  // Note: Lock should be taken before this function is called.
-  intptr_t CountScheduledLocked();
+  // Returns true if there is a rendezvous in progress and the given thread
+  // is/will be a participant of it.
+  bool IsParticipant(Thread* thread) {
+    return in_rendezvous_ && (thread->pass_safepoint_ != round_);
+  }
 
   Monitor* monitor_;  // All access is synchronized through this monitor.
   MallocGrowableArray<Entry> entries_;