Safepointing in GC

runtime/vm/thread_registry.h

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_THREAD_REGISTRY_H_
 #define VM_THREAD_REGISTRY_H_
 
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
 #include "vm/stack_frame.h"
 #include "vm/thread.h"
 
 namespace dart {
 
 // Unordered collection of threads relating to a particular isolate.
 class ThreadRegistry {
  public:
   ThreadRegistry()
       : monitor_(new Monitor()),
         entries_(),
         in_rendezvous_(false),
         remaining_(0),
         round_(0) {}
 
   // 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.
   // TODO(koda): Consider adding a scope helper to avoid omitting this call.
   void ResumeAllThreads();
 
   // Indicate that the current thread is at a safepoint, and offer to wait for
   // any pending rendezvous request (if none, returns immediately).
   void CheckSafepoint() {
     MonitorLocker ml(monitor_);
     CheckSafepointLocked();
   }
 
   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);
     if (entry != NULL) {
       Thread::State st = entry->state;
       // TODO(koda): Support same thread re-entering same isolate with
       // Dart frames in between. For now, just assert it doesn't happen.
       if (st.top_exit_frame_info != thread->top_exit_frame_info()) {
         ASSERT(thread->top_exit_frame_info() == 0 ||
                thread->top_exit_frame_info() > st.top_exit_frame_info);
@@ skipping 18 matching lines @@
     return false;
   }
 
   void SaveStateFrom(Thread* thread, const Thread::State& state) {
     MonitorLocker ml(monitor_);
     Entry* entry = FindEntry(thread);
     ASSERT(entry != NULL);
     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();
       }
     }
   }
 
   bool Contains(Thread* thread) {
     MonitorLocker ml(monitor_);
     return (FindEntry(thread) != NULL);
@@ skipping 48 matching lines @@
       if (entries_[i].thread == thread) {
         return &entries_[i];
       }
     }
     return NULL;
   }
 
   // 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_;
 
   // Safepoint rendezvous state.
   bool in_rendezvous_;    // A safepoint rendezvous request is in progress.
   intptr_t remaining_;    // Number of threads yet to reach their safepoint.
   int64_t round_;         // Counter, to prevent missing updates to remaining_
                           // (see comments in CheckSafepointLocked).
 
   DISALLOW_COPY_AND_ASSIGN(ThreadRegistry);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_REGISTRY_H_