Create ReadHolder::unlock

[folly.git] / folly / MicroSpinLock.h

diff --git a/folly/MicroSpinLock.h b/folly/MicroSpinLock.h
index 6a7588f9416b842e15436b25c901ac03df774fbc..1318d9e9f0e02d9445d17553cb4e722674eacfec 100644 (file)
--- a/folly/MicroSpinLock.h
+++ b/folly/MicroSpinLock.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 Facebook, Inc.
+ * Copyright 2016 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
 
 /*
@@ -123,11 +139,9 @@ struct FOLLY_ALIGNED_MAX SpinLockArray {
                 "Invalid size of PaddedSpinLock");
 
   // Check if T can theoretically cross a cache line.
-  // NOTE: It should be alignof(std::max_align_t), but max_align_t
-  // isn't supported by gcc 4.6.2.
-  static_assert(alignof(MaxAlign) > 0 &&
-                FOLLY_CACHE_LINE_SIZE % alignof(MaxAlign) == 0 &&
-                sizeof(T) <= alignof(MaxAlign),
+  static_assert(alignof(std::max_align_t) > 0 &&
+                FOLLY_CACHE_LINE_SIZE % alignof(std::max_align_t) == 0 &&
+                sizeof(T) <= alignof(std::max_align_t),
                 "T can cross cache line boundaries");
 
   char padding_[FOLLY_CACHE_LINE_SIZE];