Safepoint interface and unit tests.

runtime/vm/thread_registry.cc

+// 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.
+
+#include "vm/thread_registry.h"
+
+#include "vm/isolate.h"
+#include "vm/lockers.h"
+
+namespace dart {
+
+void ThreadRegistry::SafepointAllThreads() {
+  MonitorLocker ml(monitor_);
+  // First wait for any older rounds that are still in progress.
+  while (in_rendezvous_) {
+    CheckSafepointLocked();
+  }
+  // Start a new round.
+  in_rendezvous_ = true;
+  ++round_;
+  remaining_ = CountScheduledLocked();
+  Isolate* isolate = Isolate::Current();
+  // We only expect this method to be called from within the isolate itself.
+  ASSERT(isolate->thread_registry() == this);
+  // TODO(koda): Rename Thread::PrepareForGC and call it here?
+  --remaining_;  // Exclude this thread from the count.
+  // Ensure the main mutator will reach a safepoint (could be running Dart).
+  if (Thread::Current() != isolate->mutator_thread()) {
+    isolate->ScheduleInterrupts(Isolate::kSafepointInterrrupt);

[Ivan Posva, 2015/07/31 20:28:09]

Talk like a pirate constants?

[koda, 2015/07/31 22:40:44]

Rrrenamed to kVMInterrupt.

+  }
+  while (remaining_ > 0) {
+    ml.Wait(Monitor::kNoTimeout);
+  }
+}
+
+
+void ThreadRegistry::ResumeAllThreads() {
+  MonitorLocker ml(monitor_);
+  ASSERT(in_rendezvous_);
+  in_rendezvous_ = false;
+  ml.NotifyAll();
+}
+
+
+void ThreadRegistry::CheckSafepointLocked() {
+  int64_t last_round = -1;
+  while (in_rendezvous_) {
+    ASSERT(round_ >= last_round);
+    if (round_ > last_round) {
+      ASSERT((last_round == -1) || (round_ == (last_round + 1)));
+      last_round = round_;
+      // Participate in this round.
+      // TODO(koda): Rename Thread::PrepareForGC and call it here?
+      if (--remaining_ == 0) {
+        // Ensure the organizing thread is notified.
+        // TODO(koda): Use separate condition variables and plain 'Notify'.
+        monitor_->NotifyAll();

[Ivan Posva, 2015/07/31 20:28:09]

Having to use NotifyAll generally points at some problematic assumptions in the
algorithm. There should be no reason to wake up the threads that just made it
into the Wait for the safepoint to be over.

[koda, 2015/07/31 22:40:44]

I agree that it is inelegant that they all wake up, but I do not see any bugs.
The sync strategy will be revisited together with allowing threads to exit the
isolate without blocking.

+      }
+    }
+    monitor_->Wait(Monitor::kNoTimeout);
+    // Note: Here, round_ is needed to detect and distinguish two cases:
+    // a) The old rendezvous is still in progress, so just keep waiting, or
+    // b) after ResumeAllThreads, another call to SafepointAllThreads was
+    // made before this thread got a chance to reaquire monitor_, thus this
+    // thread should (again) decrease remaining_ to indicate cooperation in
+    // this new round.
+  }
+}
+
+
+intptr_t ThreadRegistry::CountScheduledLocked() {
+  intptr_t count = 0;
+  for (int i = 0; i < entries_.length(); ++i) {
+    const Entry& entry = entries_[i];
+    if (entry.scheduled) {
+      ++count;
+    }
+  }
+  return count;
+}
+
+}  // namespace dart