Explicitly handle gcc in FOLLY_MAYBE_UNUSED

[folly.git] / folly / MicroSpinLock.h

diff --git a/folly/MicroSpinLock.h b/folly/MicroSpinLock.h
index 256ab8660a878133da2e39ef45314f0e6ec0481b..ee2c4420f62a33953d68d94885708ece6cddfe0b 100644 (file)
--- a/folly/MicroSpinLock.h
+++ b/folly/MicroSpinLock.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 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
 
 /*
@@ -22,17 +38,17 @@
  */
 
 #include <array>
+#include <atomic>
 #include <cinttypes>
-#include <type_traits>
-#include <boost/noncopyable.hpp>
 #include <cstdlib>
-#include <pthread.h>
 #include <mutex>
-#include <atomic>
+#include <type_traits>
 
+#include <boost/noncopyable.hpp>
 #include <glog/logging.h>
-#include <folly/detail/Sleeper.h>
+
 #include <folly/Portability.h>
+#include <folly/detail/Sleeper.h>
 
 namespace folly {
 
@@ -89,6 +105,9 @@ struct MicroSpinLock {
                                                         std::memory_order_relaxed);
   }
 };
+static_assert(
+    std::is_pod<MicroSpinLock>::value,
+    "MicroSpinLock must be kept a POD type.");
 
 //////////////////////////////////////////////////////////////////////
 
@@ -123,10 +142,11 @@ struct FOLLY_ALIGNED_MAX SpinLockArray {
                 "Invalid size of PaddedSpinLock");
 
   // Check if T can theoretically cross a cache line.
-  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");
+  static_assert(
+      folly::max_align_v > 0 &&
+          FOLLY_CACHE_LINE_SIZE % folly::max_align_v == 0 &&
+          sizeof(T) <= folly::max_align_v,
+      "T can cross cache line boundaries");
 
   char padding_[FOLLY_CACHE_LINE_SIZE];
   std::array<PaddedSpinLock, N> data_;