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* Copyright (c) Facebook, Inc. and its affiliates.
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#pragma once
#include <cassert>
#include <climits>
#include <cstdint>
#include <folly/Portability.h>
#include <folly/detail/Futex.h>
namespace folly {
* Tiny exclusive lock that packs four lock slots into a single
* byte. Each slot is an independent real, sleeping lock. The default
* lock and unlock functions operate on slot zero, which modifies only
* the low two bits of the host byte.
* You should zero-initialize the bits of a MicroLock that you intend
* to use.
* If you're not space-constrained, prefer std::mutex, which will
* likely be faster, since it has more than two bits of information to
* work with.
* You are free to put a MicroLock in a union with some other object.
* If, for example, you want to use the bottom two bits of a pointer
* as a lock, you can put a MicroLock in a union with the pointer and
* limit yourself to MicroLock slot zero, which will use the two
* least-significant bits in the bottom byte.
* (Note that such a union is safe only because MicroLock is based on
* a character type, and even under a strict interpretation of C++'s
* aliasing rules, character types may alias anything.)
* MicroLock uses a dirty trick: it actually operates on the full
* 32-bit, four-byte-aligned bit of memory into which it is embedded.
* It never modifies bits outside the ones it's defined to modify, but
* it _accesses_ all the bits in the 32-bit memory location for
* purposes of futex management.
* The MaxSpins template parameter controls the number of times we
* spin trying to acquire the lock. MaxYields controls the number of
* times we call sched_yield; once we've tried to acquire the lock
* MaxSpins + MaxYields times, we sleep on the lock futex.
* By adjusting these parameters, you can make MicroLock behave as
* much or as little like a conventional spinlock as you'd like.
* Performance
* -----------
* With the default template options, the timings for uncontended
* acquire-then-release come out as follows on Intel(R) Xeon(R) CPU
* E5-2660 0 @ 2.20GHz, in @mode/opt, as of the master tree at Tue, 01
* Mar 2016 19:48:15.
* ========================================================================
* folly/test/SmallLocksBenchmark.cpp relative time/iter iters/s
* ========================================================================
* MicroSpinLockUncontendedBenchmark 13.46ns 74.28M
* PicoSpinLockUncontendedBenchmark 14.99ns 66.71M
* MicroLockUncontendedBenchmark 27.06ns 36.96M
* StdMutexUncontendedBenchmark 25.18ns 39.72M
* VirtualFunctionCall 1.72ns 579.78M
* ========================================================================
* (The virtual dispatch benchmark is provided for scale.)
* While the uncontended case for MicroLock is competitive with the
* glibc 2.2.0 implementation of std::mutex, std::mutex is likely to be
* faster in the contended case, because we need to wake up all waiters
* when we release.
* Make sure to benchmark your particular workload.
class MicroLockCore {
uint8_t lock_;
inline detail::Futex<>* word() const; // Well, halfword on 64-bit systems
inline uint32_t baseShift(unsigned slot) const;
inline uint32_t heldBit(unsigned slot) const;
inline uint32_t waitBit(unsigned slot) const;
static void lockSlowPath(
uint32_t oldWord,
detail::Futex<>* wordPtr,
uint32_t slotHeldBit,
unsigned maxSpins,
unsigned maxYields);
FOLLY_DISABLE_ADDRESS_SANITIZER inline void unlock(unsigned slot);
inline void unlock() {
// Initializes all the slots.
inline void init() {
lock_ = 0;
inline detail::Futex<>* MicroLockCore::word() const {
uintptr_t lockptr = (uintptr_t)&lock_;
lockptr &= ~(sizeof(uint32_t) - 1);
return (detail::Futex<>*)lockptr;
inline unsigned MicroLockCore::baseShift(unsigned slot) const {
assert(slot < CHAR_BIT / 2);
unsigned offset_bytes = (unsigned)((uintptr_t)&lock_ - (uintptr_t)word());
return (
unsigned)(kIsLittleEndian ? offset_bytes * CHAR_BIT + slot * 2 : CHAR_BIT * (sizeof(uint32_t) - offset_bytes - 1) + slot * 2);
inline uint32_t MicroLockCore::heldBit(unsigned slot) const {
return 1U << (baseShift(slot) + 0);
inline uint32_t MicroLockCore::waitBit(unsigned slot) const {
return 1U << (baseShift(slot) + 1);
void MicroLockCore::unlock(unsigned slot) {
detail::Futex<>* wordPtr = word();
uint32_t oldWord;
uint32_t newWord;
oldWord = wordPtr->load(std::memory_order_relaxed);
do {
assert(oldWord & heldBit(slot));
newWord = oldWord & ~(heldBit(slot) | waitBit(slot));
} while (!wordPtr->compare_exchange_weak(
oldWord, newWord, std::memory_order_release, std::memory_order_relaxed));
if (oldWord & waitBit(slot)) {
detail::futexWake(wordPtr, 1, heldBit(slot));
template <unsigned MaxSpins = 1000, unsigned MaxYields = 0>
class MicroLockBase : public MicroLockCore {
FOLLY_DISABLE_ADDRESS_SANITIZER inline void lock(unsigned slot);
inline void lock() {
FOLLY_DISABLE_ADDRESS_SANITIZER inline bool try_lock(unsigned slot);
inline bool try_lock() {
return try_lock(0);
template <unsigned MaxSpins, unsigned MaxYields>
bool MicroLockBase<MaxSpins, MaxYields>::try_lock(unsigned slot) {
// N.B. You might think that try_lock is just the fast path of lock,
// but you'd be wrong. Keep in mind that other parts of our host
// word might be changing while we take the lock! We're not allowed
// to fail spuriously if the lock is in fact not held, even if other
// people are concurrently modifying other parts of the word.
// We need to loop until we either see firm evidence that somebody
// else has the lock (by looking at heldBit) or see our CAS succeed.
// A failed CAS by itself does not indicate lock-acquire failure.
detail::Futex<>* wordPtr = word();
uint32_t oldWord = wordPtr->load(std::memory_order_relaxed);
do {
if (oldWord & heldBit(slot)) {
return false;
} while (!wordPtr->compare_exchange_weak(
oldWord | heldBit(slot),
return true;
template <unsigned MaxSpins, unsigned MaxYields>
void MicroLockBase<MaxSpins, MaxYields>::lock(unsigned slot) {
static_assert(MaxSpins + MaxYields < (unsigned)-1, "overflow");
detail::Futex<>* wordPtr = word();
uint32_t oldWord;
oldWord = wordPtr->load(std::memory_order_relaxed);
if ((oldWord & heldBit(slot)) == 0 &&
oldWord | heldBit(slot),
std::memory_order_relaxed)) {
// Fast uncontended case: memory_order_acquire above is our barrier
} else {
// lockSlowPath doesn't have any slot-dependent computation; it
// just shifts the input bit. Make sure its shifting produces the
// same result a call to waitBit for our slot would.
assert(heldBit(slot) << 1 == waitBit(slot));
// lockSlowPath emits its own memory barrier
lockSlowPath(oldWord, wordPtr, heldBit(slot), MaxSpins, MaxYields);
typedef MicroLockBase<> MicroLock;
} // namespace folly
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