1/*
2 * Copyright 2016-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <cassert>
20#include <climits>
21#include <cstdint>
22
23#include <folly/Portability.h>
24#include <folly/detail/Futex.h>
25
26#if defined(__clang__)
27#define NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
28#else
29#define NO_SANITIZE_ADDRESS
30#endif
31
32namespace folly {
33
34/**
35 * Tiny exclusive lock that packs four lock slots into a single
36 * byte. Each slot is an independent real, sleeping lock. The default
37 * lock and unlock functions operate on slot zero, which modifies only
38 * the low two bits of the host byte.
39 *
40 * You should zero-initialize the bits of a MicroLock that you intend
41 * to use.
42 *
43 * If you're not space-constrained, prefer std::mutex, which will
44 * likely be faster, since it has more than two bits of information to
45 * work with.
46 *
47 * You are free to put a MicroLock in a union with some other object.
48 * If, for example, you want to use the bottom two bits of a pointer
49 * as a lock, you can put a MicroLock in a union with the pointer and
50 * limit yourself to MicroLock slot zero, which will use the two
51 * least-significant bits in the bottom byte.
52 *
53 * (Note that such a union is safe only because MicroLock is based on
54 * a character type, and even under a strict interpretation of C++'s
55 * aliasing rules, character types may alias anything.)
56 *
57 * MicroLock uses a dirty trick: it actually operates on the full
58 * 32-bit, four-byte-aligned bit of memory into which it is embedded.
59 * It never modifies bits outside the ones it's defined to modify, but
60 * it _accesses_ all the bits in the 32-bit memory location for
61 * purposes of futex management.
62 *
63 * The MaxSpins template parameter controls the number of times we
64 * spin trying to acquire the lock. MaxYields controls the number of
65 * times we call sched_yield; once we've tried to acquire the lock
66 * MaxSpins + MaxYields times, we sleep on the lock futex.
67 * By adjusting these parameters, you can make MicroLock behave as
68 * much or as little like a conventional spinlock as you'd like.
69 *
70 * Performance
71 * -----------
72 *
73 * With the default template options, the timings for uncontended
74 * acquire-then-release come out as follows on Intel(R) Xeon(R) CPU
75 * E5-2660 0 @ 2.20GHz, in @mode/opt, as of the master tree at Tue, 01
76 * Mar 2016 19:48:15.
77 *
78 * ========================================================================
79 * folly/test/SmallLocksBenchmark.cpp relative time/iter iters/s
80 * ========================================================================
81 * MicroSpinLockUncontendedBenchmark 13.46ns 74.28M
82 * PicoSpinLockUncontendedBenchmark 14.99ns 66.71M
83 * MicroLockUncontendedBenchmark 27.06ns 36.96M
84 * StdMutexUncontendedBenchmark 25.18ns 39.72M
85 * VirtualFunctionCall 1.72ns 579.78M
86 * ========================================================================
87 *
88 * (The virtual dispatch benchmark is provided for scale.)
89 *
90 * While the uncontended case for MicroLock is competitive with the
91 * glibc 2.2.0 implementation of std::mutex, std::mutex is likely to be
92 * faster in the contended case, because we need to wake up all waiters
93 * when we release.
94 *
95 * Make sure to benchmark your particular workload.
96 *
97 */
98
99class MicroLockCore {
100 protected:
101#if defined(__SANITIZE_ADDRESS__) && !defined(__clang__) && \
102 (defined(__GNUC__) || defined(__GNUG__))
103 uint32_t lock_;
104#else
105 uint8_t lock_;
106#endif
107 inline detail::Futex<>* word() const; // Well, halfword on 64-bit systems
108 inline uint32_t baseShift(unsigned slot) const;
109 inline uint32_t heldBit(unsigned slot) const;
110 inline uint32_t waitBit(unsigned slot) const;
111 static void lockSlowPath(
112 uint32_t oldWord,
113 detail::Futex<>* wordPtr,
114 uint32_t slotHeldBit,
115 unsigned maxSpins,
116 unsigned maxYields);
117
118 public:
119 inline void unlock(unsigned slot) NO_SANITIZE_ADDRESS;
120 inline void unlock() {
121 unlock(0);
122 }
123 // Initializes all the slots.
124 inline void init() {
125 lock_ = 0;
126 }
127};
128
129inline detail::Futex<>* MicroLockCore::word() const {
130 uintptr_t lockptr = (uintptr_t)&lock_;
131 lockptr &= ~(sizeof(uint32_t) - 1);
132 return (detail::Futex<>*)lockptr;
133}
134
135inline unsigned MicroLockCore::baseShift(unsigned slot) const {
136 assert(slot < CHAR_BIT / 2);
137
138 unsigned offset_bytes = (unsigned)((uintptr_t)&lock_ - (uintptr_t)word());
139
140 return (
141 unsigned)(kIsLittleEndian ? offset_bytes * CHAR_BIT + slot * 2 : CHAR_BIT * (sizeof(uint32_t) - offset_bytes - 1) + slot * 2);
142}
143
144inline uint32_t MicroLockCore::heldBit(unsigned slot) const {
145 return 1U << (baseShift(slot) + 0);
146}
147
148inline uint32_t MicroLockCore::waitBit(unsigned slot) const {
149 return 1U << (baseShift(slot) + 1);
150}
151
152void MicroLockCore::unlock(unsigned slot) {
153 detail::Futex<>* wordPtr = word();
154 uint32_t oldWord;
155 uint32_t newWord;
156
157 oldWord = wordPtr->load(std::memory_order_relaxed);
158 do {
159 assert(oldWord & heldBit(slot));
160 newWord = oldWord & ~(heldBit(slot) | waitBit(slot));
161 } while (!wordPtr->compare_exchange_weak(
162 oldWord, newWord, std::memory_order_release, std::memory_order_relaxed));
163
164 if (oldWord & waitBit(slot)) {
165 detail::futexWake(wordPtr, 1, heldBit(slot));
166 }
167}
168
169template <unsigned MaxSpins = 1000, unsigned MaxYields = 0>
170class MicroLockBase : public MicroLockCore {
171 public:
172 inline void lock(unsigned slot) NO_SANITIZE_ADDRESS;
173 inline void lock() {
174 lock(0);
175 }
176 inline bool try_lock(unsigned slot) NO_SANITIZE_ADDRESS;
177 inline bool try_lock() {
178 return try_lock(0);
179 }
180};
181
182template <unsigned MaxSpins, unsigned MaxYields>
183bool MicroLockBase<MaxSpins, MaxYields>::try_lock(unsigned slot) {
184 // N.B. You might think that try_lock is just the fast path of lock,
185 // but you'd be wrong. Keep in mind that other parts of our host
186 // word might be changing while we take the lock! We're not allowed
187 // to fail spuriously if the lock is in fact not held, even if other
188 // people are concurrently modifying other parts of the word.
189 //
190 // We need to loop until we either see firm evidence that somebody
191 // else has the lock (by looking at heldBit) or see our CAS succeed.
192 // A failed CAS by itself does not indicate lock-acquire failure.
193
194 detail::Futex<>* wordPtr = word();
195 uint32_t oldWord = wordPtr->load(std::memory_order_relaxed);
196 do {
197 if (oldWord & heldBit(slot)) {
198 return false;
199 }
200 } while (!wordPtr->compare_exchange_weak(
201 oldWord,
202 oldWord | heldBit(slot),
203 std::memory_order_acquire,
204 std::memory_order_relaxed));
205
206 return true;
207}
208
209template <unsigned MaxSpins, unsigned MaxYields>
210void MicroLockBase<MaxSpins, MaxYields>::lock(unsigned slot) {
211 static_assert(MaxSpins + MaxYields < (unsigned)-1, "overflow");
212
213 detail::Futex<>* wordPtr = word();
214 uint32_t oldWord;
215 oldWord = wordPtr->load(std::memory_order_relaxed);
216 if ((oldWord & heldBit(slot)) == 0 &&
217 wordPtr->compare_exchange_weak(
218 oldWord,
219 oldWord | heldBit(slot),
220 std::memory_order_acquire,
221 std::memory_order_relaxed)) {
222 // Fast uncontended case: memory_order_acquire above is our barrier
223 } else {
224 // lockSlowPath doesn't have any slot-dependent computation; it
225 // just shifts the input bit. Make sure its shifting produces the
226 // same result a call to waitBit for our slot would.
227 assert(heldBit(slot) << 1 == waitBit(slot));
228 // lockSlowPath emits its own memory barrier
229 lockSlowPath(oldWord, wordPtr, heldBit(slot), MaxSpins, MaxYields);
230 }
231}
232
233typedef MicroLockBase<> MicroLock;
234} // namespace folly
235