summaryrefslogtreecommitdiff
path: root/absl/base/internal/spinlock.cc
blob: 9b5ed6e472028bdeeb61f95f56b20d8faed764c1 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
// Copyright 2017 The Abseil Authors.
//
// 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
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/base/internal/spinlock.h"

#include <algorithm>
#include <atomic>
#include <limits>

#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/atomic_hook.h"
#include "absl/base/internal/cycleclock.h"
#include "absl/base/internal/spinlock_wait.h"
#include "absl/base/internal/sysinfo.h" /* For NumCPUs() */
#include "absl/base/call_once.h"

// Description of lock-word:
//  31..00: [............................3][2][1][0]
//
//     [0]: kSpinLockHeld
//     [1]: kSpinLockCooperative
//     [2]: kSpinLockDisabledScheduling
// [31..3]: ONLY kSpinLockSleeper OR
//          Wait time in cycles >> PROFILE_TIMESTAMP_SHIFT
//
// Detailed descriptions:
//
// Bit [0]: The lock is considered held iff kSpinLockHeld is set.
//
// Bit [1]: Eligible waiters (e.g. Fibers) may co-operatively reschedule when
//          contended iff kSpinLockCooperative is set.
//
// Bit [2]: This bit is exclusive from bit [1].  It is used only by a
//          non-cooperative lock.  When set, indicates that scheduling was
//          successfully disabled when the lock was acquired.  May be unset,
//          even if non-cooperative, if a ThreadIdentity did not yet exist at
//          time of acquisition.
//
// Bit [3]: If this is the only upper bit ([31..3]) set then this lock was
//          acquired without contention, however, at least one waiter exists.
//
//          Otherwise, bits [31..3] represent the time spent by the current lock
//          holder to acquire the lock.  There may be outstanding waiter(s).

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace base_internal {

ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static base_internal::AtomicHook<void (*)(
    const void *lock, int64_t wait_cycles)>
    submit_profile_data;

void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
                                         int64_t wait_cycles)) {
  submit_profile_data.Store(fn);
}

#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
// Static member variable definitions.
constexpr uint32_t SpinLock::kSpinLockHeld;
constexpr uint32_t SpinLock::kSpinLockCooperative;
constexpr uint32_t SpinLock::kSpinLockDisabledScheduling;
constexpr uint32_t SpinLock::kSpinLockSleeper;
constexpr uint32_t SpinLock::kWaitTimeMask;
#endif

// Uncommon constructors.
SpinLock::SpinLock(base_internal::SchedulingMode mode)
    : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) {
  ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
}

// Monitor the lock to see if its value changes within some time period
// (adaptive_spin_count loop iterations). The last value read from the lock
// is returned from the method.
uint32_t SpinLock::SpinLoop() {
  // We are already in the slow path of SpinLock, initialize the
  // adaptive_spin_count here.
  ABSL_CONST_INIT static absl::once_flag init_adaptive_spin_count;
  ABSL_CONST_INIT static int adaptive_spin_count = 0;
  base_internal::LowLevelCallOnce(&init_adaptive_spin_count, []() {
    adaptive_spin_count = base_internal::NumCPUs() > 1 ? 1000 : 1;
  });

  int c = adaptive_spin_count;
  uint32_t lock_value;
  do {
    lock_value = lockword_.load(std::memory_order_relaxed);
  } while ((lock_value & kSpinLockHeld) != 0 && --c > 0);
  return lock_value;
}

void SpinLock::SlowLock() {
  uint32_t lock_value = SpinLoop();
  lock_value = TryLockInternal(lock_value, 0);
  if ((lock_value & kSpinLockHeld) == 0) {
    return;
  }

  base_internal::SchedulingMode scheduling_mode;
  if ((lock_value & kSpinLockCooperative) != 0) {
    scheduling_mode = base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL;
  } else {
    scheduling_mode = base_internal::SCHEDULE_KERNEL_ONLY;
  }

  // The lock was not obtained initially, so this thread needs to wait for
  // it.  Record the current timestamp in the local variable wait_start_time
  // so the total wait time can be stored in the lockword once this thread
  // obtains the lock.
  int64_t wait_start_time = CycleClock::Now();
  uint32_t wait_cycles = 0;
  int lock_wait_call_count = 0;
  while ((lock_value & kSpinLockHeld) != 0) {
    // If the lock is currently held, but not marked as having a sleeper, mark
    // it as having a sleeper.
    if ((lock_value & kWaitTimeMask) == 0) {
      // Here, just "mark" that the thread is going to sleep.  Don't store the
      // lock wait time in the lock -- the lock word stores the amount of time
      // that the current holder waited before acquiring the lock, not the wait
      // time of any thread currently waiting to acquire it.
      if (lockword_.compare_exchange_strong(
              lock_value, lock_value | kSpinLockSleeper,
              std::memory_order_relaxed, std::memory_order_relaxed)) {
        // Successfully transitioned to kSpinLockSleeper.  Pass
        // kSpinLockSleeper to the SpinLockWait routine to properly indicate
        // the last lock_value observed.
        lock_value |= kSpinLockSleeper;
      } else if ((lock_value & kSpinLockHeld) == 0) {
        // Lock is free again, so try and acquire it before sleeping.  The
        // new lock state will be the number of cycles this thread waited if
        // this thread obtains the lock.
        lock_value = TryLockInternal(lock_value, wait_cycles);
        continue;   // Skip the delay at the end of the loop.
      } else if ((lock_value & kWaitTimeMask) == 0) {
        // The lock is still held, without a waiter being marked, but something
        // else about the lock word changed, causing our CAS to fail. For
        // example, a new lock holder may have acquired the lock with
        // kSpinLockDisabledScheduling set, whereas the previous holder had not
        // set that flag. In this case, attempt again to mark ourselves as a
        // waiter.
        continue;
      }
    }

    // SpinLockDelay() calls into fiber scheduler, we need to see
    // synchronization there to avoid false positives.
    ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
    // Wait for an OS specific delay.
    base_internal::SpinLockDelay(&lockword_, lock_value, ++lock_wait_call_count,
                                 scheduling_mode);
    ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
    // Spin again after returning from the wait routine to give this thread
    // some chance of obtaining the lock.
    lock_value = SpinLoop();
    wait_cycles = EncodeWaitCycles(wait_start_time, CycleClock::Now());
    lock_value = TryLockInternal(lock_value, wait_cycles);
  }
}

void SpinLock::SlowUnlock(uint32_t lock_value) {
  base_internal::SpinLockWake(&lockword_,
                              false);  // wake waiter if necessary

  // If our acquisition was contended, collect contentionz profile info.  We
  // reserve a unitary wait time to represent that a waiter exists without our
  // own acquisition having been contended.
  if ((lock_value & kWaitTimeMask) != kSpinLockSleeper) {
    const uint64_t wait_cycles = DecodeWaitCycles(lock_value);
    ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
    submit_profile_data(this, wait_cycles);
    ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
  }
}

// We use the upper 29 bits of the lock word to store the time spent waiting to
// acquire this lock.  This is reported by contentionz profiling.  Since the
// lower bits of the cycle counter wrap very quickly on high-frequency
// processors we divide to reduce the granularity to 2^kProfileTimestampShift
// sized units.  On a 4Ghz machine this will lose track of wait times greater
// than (2^29/4 Ghz)*128 =~ 17.2 seconds.  Such waits should be extremely rare.
static constexpr int kProfileTimestampShift = 7;

// We currently reserve the lower 3 bits.
static constexpr int kLockwordReservedShift = 3;

uint32_t SpinLock::EncodeWaitCycles(int64_t wait_start_time,
                                    int64_t wait_end_time) {
  static const int64_t kMaxWaitTime =
      std::numeric_limits<uint32_t>::max() >> kLockwordReservedShift;
  int64_t scaled_wait_time =
      (wait_end_time - wait_start_time) >> kProfileTimestampShift;

  // Return a representation of the time spent waiting that can be stored in
  // the lock word's upper bits.
  uint32_t clamped = static_cast<uint32_t>(
      std::min(scaled_wait_time, kMaxWaitTime) << kLockwordReservedShift);

  if (clamped == 0) {
    return kSpinLockSleeper;  // Just wake waiters, but don't record contention.
  }
  // Bump up value if necessary to avoid returning kSpinLockSleeper.
  const uint32_t kMinWaitTime =
      kSpinLockSleeper + (1 << kLockwordReservedShift);
  if (clamped == kSpinLockSleeper) {
    return kMinWaitTime;
  }
  return clamped;
}

uint64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) {
  // Cast to uint32_t first to ensure bits [63:32] are cleared.
  const uint64_t scaled_wait_time =
      static_cast<uint32_t>(lock_value & kWaitTimeMask);
  return scaled_wait_time << (kProfileTimestampShift - kLockwordReservedShift);
}

}  // namespace base_internal
ABSL_NAMESPACE_END
}  // namespace absl