Safe and efficient stack-limit based interrupt checking in C++.

runtime/vm/isolate_test.cc

Index: runtime/vm/isolate_test.cc
diff --git a/runtime/vm/isolate_test.cc b/runtime/vm/isolate_test.cc
index f5f0506aeaa63bf4cc3356b4c4d89f994cdab854..d0d92c8a3cae41f49b5d8e7392247d859ae662be 100644
--- a/runtime/vm/isolate_test.cc
+++ b/runtime/vm/isolate_test.cc
@@ -6,6 +6,8 @@
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/isolate.h"
+#include "vm/lockers.h"
+#include "vm/thread_pool.h"
 #include "vm/unit_test.h"
 
 namespace dart {
@@ -80,4 +82,134 @@ TEST_CASE(IsolateSpawn) {
   EXPECT_VALID(exception_result);
 }
 
+
+class InterruptChecker : public ThreadPool::Task {
+ public:
+  static const intptr_t kTaskCount;
+  static const intptr_t kIterations;
+
+  InterruptChecker(Isolate* isolate,
+                   Monitor* awake_monitor,
+                   bool* awake,
+                   Monitor* round_monitor,
+                   const intptr_t* round)
+    : isolate_(isolate),
+      awake_monitor_(awake_monitor),
+      awake_(awake),
+      round_monitor_(round_monitor),
+      round_(round) {
+  }
+
+  virtual void Run() {
+    Thread::EnterIsolateAsHelper(isolate_);
+    // Tell main thread that we are ready.
+    {
+      MonitorLocker ml(awake_monitor_);
+      ASSERT(!*awake_);
+      *awake_ = true;
+      ml.Notify();
+    }
+    for (intptr_t i = 0; i < kIterations; ++i) {
+      // Busy wait for interrupts.
+      while (!isolate_->HasInterruptsScheduled(Isolate::kVMInterrupt)) {
+        // Do nothing.
+      }
+      // Tell main thread that we observed the interrupt.
+      {
+        MonitorLocker ml(awake_monitor_);
+        ASSERT(!*awake_);
+        *awake_ = true;
+        ml.Notify();
+      }
+      // Wait for main thread to let us resume, i.e., until all tasks are here.
+      {
+        MonitorLocker ml(round_monitor_);
+        EXPECT(*round_ == i || *round_ == (i + 1));
+        while (*round_ == i) {
+          ml.Wait();
+        }
+        EXPECT(*round_ == i + 1);
+      }
+    }
+    Thread::ExitIsolateAsHelper();
+    // Use awake also to signal exit.
+    {
+      MonitorLocker ml(awake_monitor_);
+      *awake_ = true;
+      ml.Notify();
+    }
+  }
+
+ private:
+  Isolate* isolate_;
+  Monitor* awake_monitor_;
+  bool* awake_;
+  Monitor* round_monitor_;
+  const intptr_t* round_;
+};
+
+
+const intptr_t InterruptChecker::kTaskCount = 50;
+const intptr_t InterruptChecker::kIterations = 1000;
+
+
+// Waits for all tasks to set their individual flag, then clears them all.
+static void WaitForAllTasks(bool* flags, Monitor* monitor) {
+  MonitorLocker ml(monitor);
+  while (true) {
+    intptr_t count = 0;
+    for (intptr_t task = 0; task < InterruptChecker::kTaskCount; ++task) {
+      if (flags[task]) {
+        ++count;
+      }
+    }
+    if (count == InterruptChecker::kTaskCount) {
+      memset(flags, 0, sizeof(*flags) * count);
+      break;
+    } else {
+      ml.Wait();
+    }
+  }
+}
+
+// Test and document usage of Isolate::HasInterruptsScheduled.
+//
+// Go through a number of rounds of scheduling interrupts and waiting until all
+// unsynchronized busy-waiting tasks observe it (in the current implementation,
+// the exact latency depends on cache coherence). Synchronization is then used
+// to ensure that the response to the interrupt, i.e., starting a new round,
+// happens *after* the interrupt is observed. Without this synchronization, the
+// compiler and/or CPU could reorder operations to make the tasks observe the
+// round update *before* the interrupt is set.
+TEST_CASE(StackLimitInterrupts) {
+  Monitor awake_monitor;  // Synchronizes the 'awake' flags.
+  bool awake[InterruptChecker::kTaskCount];
+  memset(awake, 0, sizeof(awake));
+  Monitor round_monitor;  // Synchronizes the 'round' counter.
+  intptr_t round = 0;
+  Isolate* isolate = Thread::Current()->isolate();
+  // Start all tasks. They will busy-wait until interrupted in the first round.
+  for (intptr_t task = 0; task < InterruptChecker::kTaskCount; task++) {
+    Dart::thread_pool()->Run(new InterruptChecker(
+        isolate, &awake_monitor, &awake[task], &round_monitor, &round));
+  }
+  // Wait for all tasks to get ready for the first round.
+  WaitForAllTasks(awake, &awake_monitor);
+  for (intptr_t i = 0; i < InterruptChecker::kIterations; ++i) {
+    isolate->ScheduleInterrupts(Isolate::kVMInterrupt);
+    // Wait for all tasks to observe the interrupt.
+    WaitForAllTasks(awake, &awake_monitor);
+    // Continue with next round.
+    uword interrupts = isolate->GetAndClearInterrupts();
+    EXPECT((interrupts & Isolate::kVMInterrupt) != 0);
+    {
+      MonitorLocker ml(&round_monitor);
+      ++round;
+      ml.NotifyAll();
+    }
+  }
+  // Wait for tasks to exit cleanly.
+  WaitForAllTasks(awake, &awake_monitor);
+}
+
 }  // namespace dart