/*
- * 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.
* 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
/*
#include <type_traits>
#include <boost/noncopyable.hpp>
#include <cstdlib>
-#include <pthread.h>
#include <mutex>
#include <atomic>
std::memory_order_relaxed);
}
};
+static_assert(
+ std::is_pod<MicroSpinLock>::value,
+ "MicroSpinLock must be kept a POD type.");
//////////////////////////////////////////////////////////////////////
#define FOLLY_CACHE_LINE_SIZE 64
template <class T, size_t N>
-struct SpinLockArray {
+struct FOLLY_ALIGNED_MAX SpinLockArray {
T& operator[](size_t i) {
return data_[i].lock;
}
"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];
std::array<PaddedSpinLock, N> data_;
-} __attribute__((__aligned__));
+};
//////////////////////////////////////////////////////////////////////