X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=folly%2FMicroSpinLock.h;h=8996f8c328e1016922a2881ffa6cf98efd5c2fcc;hb=59f59178ba38bc793f809d21fc532e160ae5e34b;hp=cccba15d88b6a8523f115d42537c481d7678cdfc;hpb=406822b8462145dc26be2a1a837eda9c6706dfb2;p=folly.git

diff --git a/folly/MicroSpinLock.h b/folly/MicroSpinLock.h
index cccba15d..8996f8c3 100644
--- 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 +38,15 @@
  */
 
 #include <array>
-#include <cinttypes>
-#include <type_traits>
-#include <boost/noncopyable.hpp>
-#include <cstdlib>
-#include <pthread.h>
-#include <mutex>
 #include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <mutex>
+#include <type_traits>
 
-#include <glog/logging.h>
-#include <folly/detail/Sleeper.h>
 #include <folly/Portability.h>
+#include <folly/lang/Align.h>
+#include <folly/synchronization/detail/Sleeper.h>
 
 namespace folly {
 
@@ -70,11 +84,11 @@ struct MicroSpinLock {
         sleeper.wait();
       }
     } while (!try_lock());
-    DCHECK(payload()->load() == LOCKED);
+    assert(payload()->load() == LOCKED);
   }
 
   void unlock() {
-    CHECK(payload()->load() == LOCKED);
+    assert(payload()->load() == LOCKED);
     payload()->store(FREE, std::memory_order_release);
   }
 
@@ -89,6 +103,9 @@ struct MicroSpinLock {
                                                         std::memory_order_relaxed);
   }
 };
+static_assert(
+    std::is_pod<MicroSpinLock>::value,
+    "MicroSpinLock must be kept a POD type.");
 
 //////////////////////////////////////////////////////////////////////
 
@@ -102,7 +119,7 @@ struct MicroSpinLock {
 #define FOLLY_CACHE_LINE_SIZE 64
 
 template <class T, size_t N>
-struct SpinLockArray {
+struct alignas(max_align_v) SpinLockArray {
   T& operator[](size_t i) {
     return data_[i].lock;
   }
@@ -123,16 +140,14 @@ struct 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),
-                "T can cross cache line boundaries");
+  static_assert(
+      max_align_v > 0 && FOLLY_CACHE_LINE_SIZE % max_align_v == 0 &&
+          sizeof(T) <= max_align_v,
+      "T can cross cache line boundaries");
 
   char padding_[FOLLY_CACHE_LINE_SIZE];
   std::array<PaddedSpinLock, N> data_;
-} __attribute__((__aligned__));
+};
 
 //////////////////////////////////////////////////////////////////////
 
@@ -140,4 +155,4 @@ typedef std::lock_guard<MicroSpinLock> MSLGuard;
 
 //////////////////////////////////////////////////////////////////////
 
-}
+} // namespace folly