Minor cleanup

runtime/vm/profiler.h

Index: runtime/vm/profiler.h
diff --git a/runtime/vm/profiler.h b/runtime/vm/profiler.h
index affe7100b46a74ad9cc6c82d4489947a83a06169..ccdd9b96abd9db0f35b99e005892e6c6bd4a52ae 100644
--- a/runtime/vm/profiler.h
+++ b/runtime/vm/profiler.h
@@ -38,6 +38,15 @@ class Profiler : public AllStatic {
   }
 
   static void SampleAllocation(Thread* thread, intptr_t cid);
+
+  // SampleThread is called from inside the signal handler and hence it is very
+  // critical that the implementation of SampleThread does not do any of the
+  // following:
+  //   * Accessing TLS -- Because on Windows the callback will be running in a
+  //                      different thread.
+  //   * Allocating memory -- Because this takes locks which may already be
+  //                          held, resulting in a dead lock.
+  //   * Taking a lock -- See above.
   static void SampleThread(Thread* thread,
                            const InterruptedThreadState& state);