X-Git-Url: http://plrg.eecs.uci.edu/git/?p=folly.git;a=blobdiff_plain;f=folly%2FMicroSpinLock.h;h=0453b35599ec3e526633171f386b6285d87679c8;hp=fde93357bc610238d5ae589a4fd820358f17a7f8;hb=00ff5917775f9b58a04f74835585cbb32e306289;hpb=321542683a01c3f334047531e9b487f047129775

diff --git a/folly/MicroSpinLock.h b/folly/MicroSpinLock.h
index fde93357..0453b355 100644
--- a/folly/MicroSpinLock.h
+++ b/folly/MicroSpinLock.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,6 +14,22 @@
  * limitations under the License.
  */
 
+/*
+ * N.B. You most likely do _not_ want to use MicroSpinLock or any
+ * other kind of spinlock.  Consider MicroLock instead.
+ *
+ * In short, spinlocks in preemptive multi-tasking operating systems
+ * have serious problems and fast mutexes like std::mutex are almost
+ * certainly the better choice, because letting the OS scheduler put a
+ * thread to sleep is better for system responsiveness and throughput
+ * than wasting a timeslice repeatedly querying a lock held by a
+ * thread that's blocked, and you can't prevent userspace
+ * programs blocking.
+ *
+ * Spinlocks in an operating system kernel make much more sense than
+ * they do in userspace.
+ */
+
 #pragma once
 
 /*
@@ -26,7 +42,6 @@
 #include <type_traits>
 #include <boost/noncopyable.hpp>
 #include <cstdlib>
-#include <pthread.h>
 #include <mutex>
 #include <atomic>
 
@@ -89,6 +104,9 @@ struct MicroSpinLock {
                                                         std::memory_order_relaxed);
   }
 };
+static_assert(
+    std::is_pod<MicroSpinLock>::value,
+    "MicroSpinLock must be kept a POD type.");
 
 //////////////////////////////////////////////////////////////////////