1 //
2 // Copyright 2017 The Abseil Authors.
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 // https://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 // Most users requiring mutual exclusion should use Mutex.
18 // SpinLock is provided for use in three situations:
19 // - for use in code that Mutex itself depends on
20 // - to get a faster fast-path release under low contention (without an
21 // atomic read-modify-write) In return, SpinLock has worse behaviour under
22 // contention, which is why Mutex is preferred in most situations.
23 // - for async signal safety (see below)
24
25 // SpinLock is async signal safe. If a spinlock is used within a signal
26 // handler, all code that acquires the lock must ensure that the signal cannot
27 // arrive while they are holding the lock. Typically, this is done by blocking
28 // the signal.
29
30 #ifndef ABSL_BASE_INTERNAL_SPINLOCK_H_
31 #define ABSL_BASE_INTERNAL_SPINLOCK_H_
32
33 #include <stdint.h>
34 #include <sys/types.h>
35
36 #include <atomic>
37
38 #include "absl/base/attributes.h"
39 #include "absl/base/const_init.h"
40 #include "absl/base/dynamic_annotations.h"
41 #include "absl/base/internal/low_level_scheduling.h"
42 #include "absl/base/internal/raw_logging.h"
43 #include "absl/base/internal/scheduling_mode.h"
44 #include "absl/base/internal/tsan_mutex_interface.h"
45 #include "absl/base/macros.h"
46 #include "absl/base/port.h"
47 #include "absl/base/thread_annotations.h"
48
49 namespace absl {
50 ABSL_NAMESPACE_BEGIN
51 namespace base_internal {
52
53 class ABSL_LOCKABLE SpinLock {
54 public:
SpinLock()55 SpinLock() : lockword_(kSpinLockCooperative) {
56 ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
57 }
58
59 // Constructors that allow non-cooperative spinlocks to be created for use
60 // inside thread schedulers. Normal clients should not use these.
61 explicit SpinLock(base_internal::SchedulingMode mode);
62
63 // Constructor for global SpinLock instances. See absl/base/const_init.h.
SpinLock(absl::ConstInitType,base_internal::SchedulingMode mode)64 constexpr SpinLock(absl::ConstInitType, base_internal::SchedulingMode mode)
65 : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) {}
66
67 // For global SpinLock instances prefer trivial destructor when possible.
68 // Default but non-trivial destructor in some build configurations causes an
69 // extra static initializer.
70 #ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
~SpinLock()71 ~SpinLock() { ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static); }
72 #else
73 ~SpinLock() = default;
74 #endif
75
76 // Acquire this SpinLock.
Lock()77 inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() {
78 ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
79 if (!TryLockImpl()) {
80 SlowLock();
81 }
82 ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
83 }
84
85 // Try to acquire this SpinLock without blocking and return true if the
86 // acquisition was successful. If the lock was not acquired, false is
87 // returned. If this SpinLock is free at the time of the call, TryLock
88 // will return true with high probability.
TryLock()89 inline bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
90 ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
91 bool res = TryLockImpl();
92 ABSL_TSAN_MUTEX_POST_LOCK(
93 this, __tsan_mutex_try_lock | (res ? 0 : __tsan_mutex_try_lock_failed),
94 0);
95 return res;
96 }
97
98 // Release this SpinLock, which must be held by the calling thread.
Unlock()99 inline void Unlock() ABSL_UNLOCK_FUNCTION() {
100 ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
101 uint32_t lock_value = lockword_.load(std::memory_order_relaxed);
102 lock_value = lockword_.exchange(lock_value & kSpinLockCooperative,
103 std::memory_order_release);
104
105 if ((lock_value & kSpinLockDisabledScheduling) != 0) {
106 base_internal::SchedulingGuard::EnableRescheduling(true);
107 }
108 if ((lock_value & kWaitTimeMask) != 0) {
109 // Collect contentionz profile info, and speed the wakeup of any waiter.
110 // The wait_cycles value indicates how long this thread spent waiting
111 // for the lock.
112 SlowUnlock(lock_value);
113 }
114 ABSL_TSAN_MUTEX_POST_UNLOCK(this, 0);
115 }
116
117 // Determine if the lock is held. When the lock is held by the invoking
118 // thread, true will always be returned. Intended to be used as
119 // CHECK(lock.IsHeld()).
IsHeld()120 inline bool IsHeld() const {
121 return (lockword_.load(std::memory_order_relaxed) & kSpinLockHeld) != 0;
122 }
123
124 protected:
125 // These should not be exported except for testing.
126
127 // Store number of cycles between wait_start_time and wait_end_time in a
128 // lock value.
129 static uint32_t EncodeWaitCycles(int64_t wait_start_time,
130 int64_t wait_end_time);
131
132 // Extract number of wait cycles in a lock value.
133 static uint64_t DecodeWaitCycles(uint32_t lock_value);
134
135 // Provide access to protected method above. Use for testing only.
136 friend struct SpinLockTest;
137
138 private:
139 // lockword_ is used to store the following:
140 //
141 // bit[0] encodes whether a lock is being held.
142 // bit[1] encodes whether a lock uses cooperative scheduling.
143 // bit[2] encodes whether a lock disables scheduling.
144 // bit[3:31] encodes time a lock spent on waiting as a 29-bit unsigned int.
145 static constexpr uint32_t kSpinLockHeld = 1;
146 static constexpr uint32_t kSpinLockCooperative = 2;
147 static constexpr uint32_t kSpinLockDisabledScheduling = 4;
148 static constexpr uint32_t kSpinLockSleeper = 8;
149 // Includes kSpinLockSleeper.
150 static constexpr uint32_t kWaitTimeMask =
151 ~(kSpinLockHeld | kSpinLockCooperative | kSpinLockDisabledScheduling);
152
153 // Returns true if the provided scheduling mode is cooperative.
IsCooperative(base_internal::SchedulingMode scheduling_mode)154 static constexpr bool IsCooperative(
155 base_internal::SchedulingMode scheduling_mode) {
156 return scheduling_mode == base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL;
157 }
158
159 uint32_t TryLockInternal(uint32_t lock_value, uint32_t wait_cycles);
160 void SlowLock() ABSL_ATTRIBUTE_COLD;
161 void SlowUnlock(uint32_t lock_value) ABSL_ATTRIBUTE_COLD;
162 uint32_t SpinLoop();
163
TryLockImpl()164 inline bool TryLockImpl() {
165 uint32_t lock_value = lockword_.load(std::memory_order_relaxed);
166 return (TryLockInternal(lock_value, 0) & kSpinLockHeld) == 0;
167 }
168
169 std::atomic<uint32_t> lockword_;
170
171 SpinLock(const SpinLock&) = delete;
172 SpinLock& operator=(const SpinLock&) = delete;
173 };
174
175 // Corresponding locker object that arranges to acquire a spinlock for
176 // the duration of a C++ scope.
177 class ABSL_SCOPED_LOCKABLE SpinLockHolder {
178 public:
SpinLockHolder(SpinLock * l)179 inline explicit SpinLockHolder(SpinLock* l) ABSL_EXCLUSIVE_LOCK_FUNCTION(l)
180 : lock_(l) {
181 l->Lock();
182 }
ABSL_UNLOCK_FUNCTION()183 inline ~SpinLockHolder() ABSL_UNLOCK_FUNCTION() { lock_->Unlock(); }
184
185 SpinLockHolder(const SpinLockHolder&) = delete;
186 SpinLockHolder& operator=(const SpinLockHolder&) = delete;
187
188 private:
189 SpinLock* lock_;
190 };
191
192 // Register a hook for profiling support.
193 //
194 // The function pointer registered here will be called whenever a spinlock is
195 // contended. The callback is given an opaque handle to the contended spinlock
196 // and the number of wait cycles. This is thread-safe, but only a single
197 // profiler can be registered. It is an error to call this function multiple
198 // times with different arguments.
199 void RegisterSpinLockProfiler(void (*fn)(const void* lock,
200 int64_t wait_cycles));
201
202 //------------------------------------------------------------------------------
203 // Public interface ends here.
204 //------------------------------------------------------------------------------
205
206 // If (result & kSpinLockHeld) == 0, then *this was successfully locked.
207 // Otherwise, returns last observed value for lockword_.
TryLockInternal(uint32_t lock_value,uint32_t wait_cycles)208 inline uint32_t SpinLock::TryLockInternal(uint32_t lock_value,
209 uint32_t wait_cycles) {
210 if ((lock_value & kSpinLockHeld) != 0) {
211 return lock_value;
212 }
213
214 uint32_t sched_disabled_bit = 0;
215 if ((lock_value & kSpinLockCooperative) == 0) {
216 // For non-cooperative locks we must make sure we mark ourselves as
217 // non-reschedulable before we attempt to CompareAndSwap.
218 if (base_internal::SchedulingGuard::DisableRescheduling()) {
219 sched_disabled_bit = kSpinLockDisabledScheduling;
220 }
221 }
222
223 if (!lockword_.compare_exchange_strong(
224 lock_value,
225 kSpinLockHeld | lock_value | wait_cycles | sched_disabled_bit,
226 std::memory_order_acquire, std::memory_order_relaxed)) {
227 base_internal::SchedulingGuard::EnableRescheduling(sched_disabled_bit != 0);
228 }
229
230 return lock_value;
231 }
232
233 } // namespace base_internal
234 ABSL_NAMESPACE_END
235 } // namespace absl
236
237 #endif // ABSL_BASE_INTERNAL_SPINLOCK_H_
238