Ewout van Bekkum | 5890193 | 2020-11-09 12:46:52 -0800 | [diff] [blame] | 1 | .. _module-pw_sync: |
| 2 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 3 | ======= |
Ewout van Bekkum | 5890193 | 2020-11-09 12:46:52 -0800 | [diff] [blame] | 4 | pw_sync |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 5 | ======= |
| 6 | The ``pw_sync`` module contains utilities for synchronizing between threads |
| 7 | and/or interrupts through signaling primitives and critical section lock |
| 8 | primitives. |
Ewout van Bekkum | 5890193 | 2020-11-09 12:46:52 -0800 | [diff] [blame] | 9 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 10 | .. Warning:: |
| 11 | This module is still under construction, the API is not yet stable. |
| 12 | |
| 13 | .. Note:: |
| 14 | The objects in this module do not have an Init() style public API which is |
| 15 | common in many RTOS C APIs. Instead, they rely on being able to invoke the |
| 16 | native initialization APIs for synchronization primitives during C++ |
| 17 | construction. |
| 18 | In order to support global statically constructed synchronization without |
| 19 | constexpr constructors, the user and/or backend **MUST** ensure that any |
| 20 | initialization required in your environment is done prior to the creation |
| 21 | and/or initialization of the native synchronization primitives |
| 22 | (e.g. kernel initialization). |
| 23 | |
| 24 | -------------------------------- |
| 25 | Critical Section Lock Primitives |
| 26 | -------------------------------- |
| 27 | The critical section lock primitives provided by this module comply with |
| 28 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_, |
| 29 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_, and where |
| 30 | relevant |
| 31 | `TimedLockable <https://en.cppreference.com/w/cpp/named_req/TimedLockable>`_ C++ |
| 32 | named requirements. This means that they are compatible with existing helpers in |
| 33 | the STL's ``<mutex>`` thread support library. For example `std::lock_guard <https://en.cppreference.com/w/cpp/thread/lock_guard>`_ |
| 34 | and `std::unique_lock <https://en.cppreference.com/w/cpp/thread/unique_lock>`_ can be directly used. |
| 35 | |
| 36 | Mutex |
| 37 | ===== |
| 38 | The Mutex is a synchronization primitive that can be used to protect shared data |
| 39 | from being simultaneously accessed by multiple threads. It offers exclusive, |
| 40 | non-recursive ownership semantics where priority inheritance is used to solve |
| 41 | the classic priority-inversion problem. |
| 42 | |
| 43 | The Mutex's API is C++11 STL |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 44 | `std::mutex <https://en.cppreference.com/w/cpp/thread/mutex>`_ like, |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 45 | meaning it is a |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 46 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_ |
| 47 | and `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_. |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 48 | |
| 49 | .. list-table:: |
| 50 | |
| 51 | * - *Supported on* |
| 52 | - *Backend module* |
| 53 | * - FreeRTOS |
| 54 | - :ref:`module-pw_sync_freertos` |
| 55 | * - ThreadX |
| 56 | - :ref:`module-pw_sync_threadx` |
| 57 | * - embOS |
| 58 | - :ref:`module-pw_sync_embos` |
| 59 | * - STL |
| 60 | - :ref:`module-pw_sync_stl` |
| 61 | * - Baremetal |
| 62 | - Planned |
| 63 | * - Zephyr |
| 64 | - Planned |
| 65 | * - CMSIS-RTOS API v2 & RTX5 |
| 66 | - Planned |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 67 | |
| 68 | C++ |
| 69 | --- |
| 70 | .. cpp:class:: pw::sync::Mutex |
| 71 | |
| 72 | .. cpp:function:: void lock() |
| 73 | |
| 74 | Locks the mutex, blocking indefinitely. Failures are fatal. |
| 75 | |
| 76 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 77 | locking is undefined behavior. |
| 78 | |
| 79 | .. cpp:function:: bool try_lock() |
| 80 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 81 | Tries to lock the mutex in a non-blocking manner. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 82 | Returns true if the mutex was successfully acquired. |
| 83 | |
| 84 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 85 | locking is undefined behavior. |
| 86 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 87 | .. cpp:function:: void unlock() |
| 88 | |
| 89 | Unlocks the mutex. Failures are fatal. |
| 90 | |
| 91 | **Precondition:** The mutex is held by this thread. |
| 92 | |
| 93 | |
| 94 | .. list-table:: |
| 95 | |
| 96 | * - *Safe to use in context* |
| 97 | - *Thread* |
| 98 | - *Interrupt* |
| 99 | - *NMI* |
| 100 | * - ``Mutex::Mutex`` |
| 101 | - ✔ |
| 102 | - |
| 103 | - |
| 104 | * - ``Mutex::~Mutex`` |
| 105 | - ✔ |
| 106 | - |
| 107 | - |
| 108 | * - ``void Mutex::lock`` |
| 109 | - ✔ |
| 110 | - |
| 111 | - |
| 112 | * - ``bool Mutex::try_lock`` |
| 113 | - ✔ |
| 114 | - |
| 115 | - |
| 116 | * - ``void Mutex::unlock`` |
| 117 | - ✔ |
| 118 | - |
| 119 | - |
| 120 | |
| 121 | Examples in C++ |
| 122 | ^^^^^^^^^^^^^^^ |
| 123 | .. code-block:: cpp |
| 124 | |
| 125 | #include "pw_sync/mutex.h" |
| 126 | |
| 127 | pw::sync::Mutex mutex; |
| 128 | |
| 129 | void ThreadSafeCriticalSection() { |
| 130 | mutex.lock(); |
| 131 | NotThreadSafeCriticalSection(); |
| 132 | mutex.unlock(); |
| 133 | } |
| 134 | |
| 135 | |
| 136 | Alternatively you can use C++'s RAII helpers to ensure you always unlock. |
| 137 | |
| 138 | .. code-block:: cpp |
| 139 | |
| 140 | #include <mutex> |
| 141 | |
| 142 | #include "pw_sync/mutex.h" |
| 143 | |
| 144 | pw::sync::Mutex mutex; |
| 145 | |
| 146 | void ThreadSafeCriticalSection() { |
| 147 | std::lock_guard lock(mutex); |
| 148 | NotThreadSafeCriticalSection(); |
| 149 | } |
| 150 | |
| 151 | |
| 152 | C |
| 153 | - |
| 154 | The Mutex must be created in C++, however it can be passed into C using the |
| 155 | ``pw_sync_Mutex`` opaque struct alias. |
| 156 | |
| 157 | .. cpp:function:: void pw_sync_Mutex_Lock(pw_sync_Mutex* mutex) |
| 158 | |
| 159 | Invokes the ``Mutex::lock`` member function on the given ``mutex``. |
| 160 | |
| 161 | .. cpp:function:: bool pw_sync_Mutex_TryLock(pw_sync_Mutex* mutex) |
| 162 | |
| 163 | Invokes the ``Mutex::try_lock`` member function on the given ``mutex``. |
| 164 | |
| 165 | .. cpp:function:: void pw_sync_Mutex_Unlock(pw_sync_Mutex* mutex) |
| 166 | |
| 167 | Invokes the ``Mutex::unlock`` member function on the given ``mutex``. |
| 168 | |
| 169 | .. list-table:: |
| 170 | |
| 171 | * - *Safe to use in context* |
| 172 | - *Thread* |
| 173 | - *Interrupt* |
| 174 | - *NMI* |
| 175 | * - ``void pw_sync_Mutex_Lock`` |
| 176 | - ✔ |
| 177 | - |
| 178 | - |
| 179 | * - ``bool pw_sync_Mutex_TryLock`` |
| 180 | - ✔ |
| 181 | - |
| 182 | - |
| 183 | * - ``void pw_sync_Mutex_Unlock`` |
| 184 | - ✔ |
| 185 | - |
| 186 | - |
| 187 | |
| 188 | Example in C |
| 189 | ^^^^^^^^^^^^ |
| 190 | .. code-block:: cpp |
| 191 | |
| 192 | #include "pw_sync/mutex.h" |
| 193 | |
| 194 | pw::sync::Mutex mutex; |
| 195 | |
| 196 | extern pw_sync_Mutex mutex; // This can only be created in C++. |
| 197 | |
| 198 | void ThreadSafeCriticalSection(void) { |
| 199 | pw_sync_Mutex_Lock(&mutex); |
| 200 | NotThreadSafeCriticalSection(); |
| 201 | pw_sync_Mutex_Unlock(&mutex); |
| 202 | } |
| 203 | |
| 204 | TimedMutex |
| 205 | ========== |
| 206 | The TimedMutex is an extension of the Mutex which offers timeout and deadline |
| 207 | based semantics. |
| 208 | |
| 209 | The TimedMutex's API is C++11 STL |
| 210 | `std::timed_mutex <https://en.cppreference.com/w/cpp/thread/timed_mutex>`_ like, |
| 211 | meaning it is a |
| 212 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_, |
| 213 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_, and |
| 214 | `TimedLockable <https://en.cppreference.com/w/cpp/named_req/TimedLockable>`_. |
| 215 | |
| 216 | Note that the ``TimedMutex`` is a derived ``Mutex`` class, meaning that |
| 217 | a ``TimedMutex`` can be used by someone who needs the basic ``Mutex``. This is |
Ewout van Bekkum | 0bf9625 | 2021-08-24 09:44:06 -0700 | [diff] [blame] | 218 | in contrast to the C++ STL's |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 219 | `std::timed_mutex <https://en.cppreference.com/w/cpp/thread/timed_mutex>`_. |
| 220 | |
| 221 | |
| 222 | .. list-table:: |
| 223 | |
| 224 | * - *Supported on* |
| 225 | - *Backend module* |
| 226 | * - FreeRTOS |
| 227 | - :ref:`module-pw_sync_freertos` |
| 228 | * - ThreadX |
| 229 | - :ref:`module-pw_sync_threadx` |
| 230 | * - embOS |
| 231 | - :ref:`module-pw_sync_embos` |
| 232 | * - STL |
| 233 | - :ref:`module-pw_sync_stl` |
| 234 | * - Zephyr |
| 235 | - Planned |
| 236 | * - CMSIS-RTOS API v2 & RTX5 |
| 237 | - Planned |
| 238 | |
| 239 | C++ |
| 240 | --- |
| 241 | .. cpp:class:: pw::sync::TimedMutex |
| 242 | |
| 243 | .. cpp:function:: void lock() |
| 244 | |
| 245 | Locks the mutex, blocking indefinitely. Failures are fatal. |
| 246 | |
| 247 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 248 | locking is undefined behavior. |
| 249 | |
| 250 | .. cpp:function:: bool try_lock() |
| 251 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 252 | Tries to lock the mutex in a non-blocking manner. |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 253 | Returns true if the mutex was successfully acquired. |
| 254 | |
| 255 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 256 | locking is undefined behavior. |
| 257 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 258 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 259 | .. cpp:function:: bool try_lock_for(const chrono::SystemClock::duration& timeout) |
| 260 | |
| 261 | Tries to lock the mutex. Blocks until specified the timeout has elapsed or |
| 262 | the lock is acquired, whichever comes first. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 263 | Returns true if the mutex was successfully acquired. |
| 264 | |
| 265 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 266 | locking is undefined behavior. |
| 267 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 268 | .. cpp:function:: bool try_lock_until(const chrono::SystemClock::time_point& deadline) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 269 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 270 | Tries to lock the mutex. Blocks until specified deadline has been reached |
| 271 | or the lock is acquired, whichever comes first. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 272 | Returns true if the mutex was successfully acquired. |
| 273 | |
| 274 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 275 | locking is undefined behavior. |
| 276 | |
| 277 | .. cpp:function:: void unlock() |
| 278 | |
| 279 | Unlocks the mutex. Failures are fatal. |
| 280 | |
| 281 | **Precondition:** The mutex is held by this thread. |
| 282 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 283 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 284 | .. list-table:: |
| 285 | |
| 286 | * - *Safe to use in context* |
| 287 | - *Thread* |
| 288 | - *Interrupt* |
| 289 | - *NMI* |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 290 | * - ``TimedMutex::TimedMutex`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 291 | - ✔ |
| 292 | - |
| 293 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 294 | * - ``TimedMutex::~TimedMutex`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 295 | - ✔ |
| 296 | - |
| 297 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 298 | * - ``void TimedMutex::lock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 299 | - ✔ |
| 300 | - |
| 301 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 302 | * - ``bool TimedMutex::try_lock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 303 | - ✔ |
| 304 | - |
| 305 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 306 | * - ``bool TimedMutex::try_lock_for`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 307 | - ✔ |
| 308 | - |
| 309 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 310 | * - ``bool TimedMutex::try_lock_until`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 311 | - ✔ |
| 312 | - |
| 313 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 314 | * - ``void TimedMutex::unlock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 315 | - ✔ |
| 316 | - |
| 317 | - |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 318 | |
| 319 | Examples in C++ |
| 320 | ^^^^^^^^^^^^^^^ |
| 321 | .. code-block:: cpp |
| 322 | |
| 323 | #include "pw_chrono/system_clock.h" |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 324 | #include "pw_sync/timed_mutex.h" |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 325 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 326 | pw::sync::TimedMutex mutex; |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 327 | |
| 328 | bool ThreadSafeCriticalSectionWithTimeout( |
| 329 | const SystemClock::duration timeout) { |
| 330 | if (!mutex.try_lock_for(timeout)) { |
| 331 | return false; |
| 332 | } |
| 333 | NotThreadSafeCriticalSection(); |
| 334 | mutex.unlock(); |
| 335 | return true; |
| 336 | } |
| 337 | |
| 338 | |
| 339 | Alternatively you can use C++'s RAII helpers to ensure you always unlock. |
| 340 | |
| 341 | .. code-block:: cpp |
| 342 | |
| 343 | #include <mutex> |
| 344 | |
| 345 | #include "pw_chrono/system_clock.h" |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 346 | #include "pw_sync/timed_mutex.h" |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 347 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 348 | pw::sync::TimedMutex mutex; |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 349 | |
| 350 | bool ThreadSafeCriticalSectionWithTimeout( |
| 351 | const SystemClock::duration timeout) { |
| 352 | std::unique_lock lock(mutex, std::defer_lock); |
| 353 | if (!lock.try_lock_for(timeout)) { |
| 354 | return false; |
| 355 | } |
| 356 | NotThreadSafeCriticalSection(); |
| 357 | return true; |
| 358 | } |
| 359 | |
| 360 | |
| 361 | |
| 362 | C |
| 363 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 364 | The TimedMutex must be created in C++, however it can be passed into C using the |
| 365 | ``pw_sync_TimedMutex`` opaque struct alias. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 366 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 367 | .. cpp:function:: void pw_sync_TimedMutex_Lock(pw_sync_TimedMutex* mutex) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 368 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 369 | Invokes the ``TimedMutex::lock`` member function on the given ``mutex``. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 370 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 371 | .. cpp:function:: bool pw_sync_TimedMutex_TryLock(pw_sync_TimedMutex* mutex) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 372 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 373 | Invokes the ``TimedMutex::try_lock`` member function on the given ``mutex``. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 374 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 375 | .. cpp:function:: bool pw_sync_TimedMutex_TryLockFor(pw_sync_TimedMutex* mutex, pw_chrono_SystemClock_Duration timeout) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 376 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 377 | Invokes the ``TimedMutex::try_lock_for`` member function on the given ``mutex``. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 378 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 379 | .. cpp:function:: bool pw_sync_TimedMutex_TryLockUntil(pw_sync_TimedMutex* mutex, pw_chrono_SystemClock_TimePoint deadline) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 380 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 381 | Invokes the ``TimedMutex::try_lock_until`` member function on the given ``mutex``. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 382 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 383 | .. cpp:function:: void pw_sync_TimedMutex_Unlock(pw_sync_TimedMutex* mutex) |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 384 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 385 | Invokes the ``TimedMutex::unlock`` member function on the given ``mutex``. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 386 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 387 | .. list-table:: |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 388 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 389 | * - *Safe to use in context* |
| 390 | - *Thread* |
| 391 | - *Interrupt* |
| 392 | - *NMI* |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 393 | * - ``void pw_sync_TimedMutex_Lock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 394 | - ✔ |
| 395 | - |
| 396 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 397 | * - ``bool pw_sync_TimedMutex_TryLock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 398 | - ✔ |
| 399 | - |
| 400 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 401 | * - ``bool pw_sync_TimedMutex_TryLockFor`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 402 | - ✔ |
| 403 | - |
| 404 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 405 | * - ``bool pw_sync_TimedMutex_TryLockUntil`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 406 | - ✔ |
| 407 | - |
| 408 | - |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 409 | * - ``void pw_sync_TimedMutex_Unlock`` |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 410 | - ✔ |
| 411 | - |
| 412 | - |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 413 | |
| 414 | Example in C |
| 415 | ^^^^^^^^^^^^ |
| 416 | .. code-block:: cpp |
| 417 | |
| 418 | #include "pw_chrono/system_clock.h" |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 419 | #include "pw_sync/timed_mutex.h" |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 420 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 421 | pw::sync::TimedMutex mutex; |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 422 | |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 423 | extern pw_sync_TimedMutex mutex; // This can only be created in C++. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 424 | |
| 425 | bool ThreadSafeCriticalSectionWithTimeout( |
| 426 | const pw_chrono_SystemClock_Duration timeout) { |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 427 | if (!pw_sync_TimedMutex_TryLockFor(&mutex, timeout)) { |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 428 | return false; |
| 429 | } |
| 430 | NotThreadSafeCriticalSection(); |
Ewout van Bekkum | 6f5b8fb | 2021-04-06 16:15:22 -0700 | [diff] [blame] | 431 | pw_sync_TimedMutex_Unlock(&mutex); |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 432 | return true; |
| 433 | } |
| 434 | |
| 435 | |
| 436 | InterruptSpinLock |
| 437 | ================= |
| 438 | The InterruptSpinLock is a synchronization primitive that can be used to protect |
| 439 | shared data from being simultaneously accessed by multiple threads and/or |
| 440 | interrupts as a targeted global lock, with the exception of Non-Maskable |
| 441 | Interrupts (NMIs). It offers exclusive, non-recursive ownership semantics where |
| 442 | IRQs up to a backend defined level of "NMIs" will be masked to solve |
| 443 | priority-inversion. |
| 444 | |
| 445 | This InterruptSpinLock relies on built-in local interrupt masking to make it |
| 446 | interrupt safe without requiring the caller to separately mask and unmask |
| 447 | interrupts when using this primitive. |
| 448 | |
| 449 | Unlike global interrupt locks, this also works safely and efficiently on SMP |
| 450 | systems. On systems which are not SMP, spinning is not required but some state |
| 451 | may still be used to detect recursion. |
| 452 | |
| 453 | The InterruptSpinLock is a |
| 454 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_ |
| 455 | and |
| 456 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_. |
| 457 | |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 458 | .. list-table:: |
| 459 | |
| 460 | * - *Supported on* |
| 461 | - *Backend module* |
| 462 | * - FreeRTOS |
| 463 | - :ref:`module-pw_sync_freertos` |
| 464 | * - ThreadX |
| 465 | - :ref:`module-pw_sync_threadx` |
| 466 | * - embOS |
| 467 | - :ref:`module-pw_sync_embos` |
| 468 | * - STL |
| 469 | - :ref:`module-pw_sync_stl` |
| 470 | * - Baremetal |
| 471 | - Planned, not ready for use |
| 472 | * - Zephyr |
| 473 | - Planned |
| 474 | * - CMSIS-RTOS API v2 & RTX5 |
| 475 | - Planned |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 476 | |
| 477 | C++ |
| 478 | --- |
| 479 | .. cpp:class:: pw::sync::InterruptSpinLock |
| 480 | |
| 481 | .. cpp:function:: void lock() |
| 482 | |
| 483 | Locks the spinlock, blocking indefinitely. Failures are fatal. |
| 484 | |
| 485 | **Precondition:** Recursive locking is undefined behavior. |
| 486 | |
| 487 | .. cpp:function:: bool try_lock() |
| 488 | |
Ewout van Bekkum | 5ff8cc5 | 2021-09-07 15:46:36 -0700 | [diff] [blame^] | 489 | Tries to lock the spinlock in a non-blocking manner. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 490 | Returns true if the spinlock was successfully acquired. |
| 491 | |
| 492 | **Precondition:** Recursive locking is undefined behavior. |
| 493 | |
| 494 | .. cpp:function:: void unlock() |
| 495 | |
| 496 | Unlocks the mutex. Failures are fatal. |
| 497 | |
| 498 | **Precondition:** The spinlock is held by the caller. |
| 499 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 500 | .. list-table:: |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 501 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 502 | * - *Safe to use in context* |
| 503 | - *Thread* |
| 504 | - *Interrupt* |
| 505 | - *NMI* |
| 506 | * - ``InterruptSpinLock::InterruptSpinLock`` |
| 507 | - ✔ |
| 508 | - ✔ |
| 509 | - |
| 510 | * - ``InterruptSpinLock::~InterruptSpinLock`` |
| 511 | - ✔ |
| 512 | - ✔ |
| 513 | - |
| 514 | * - ``void InterruptSpinLock::lock`` |
| 515 | - ✔ |
| 516 | - ✔ |
| 517 | - |
| 518 | * - ``bool InterruptSpinLock::try_lock`` |
| 519 | - ✔ |
| 520 | - ✔ |
| 521 | - |
| 522 | * - ``void InterruptSpinLock::unlock`` |
| 523 | - ✔ |
| 524 | - ✔ |
| 525 | - |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 526 | |
| 527 | Examples in C++ |
| 528 | ^^^^^^^^^^^^^^^ |
| 529 | .. code-block:: cpp |
| 530 | |
| 531 | #include "pw_sync/interrupt_spin_lock.h" |
| 532 | |
| 533 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 534 | |
| 535 | void InterruptSafeCriticalSection() { |
| 536 | interrupt_spin_lock.lock(); |
| 537 | NotThreadSafeCriticalSection(); |
| 538 | interrupt_spin_lock.unlock(); |
| 539 | } |
| 540 | |
| 541 | |
| 542 | Alternatively you can use C++'s RAII helpers to ensure you always unlock. |
| 543 | |
| 544 | .. code-block:: cpp |
| 545 | |
| 546 | #include <mutex> |
| 547 | |
| 548 | #include "pw_sync/interrupt_spin_lock.h" |
| 549 | |
| 550 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 551 | |
| 552 | void InterruptSafeCriticalSection() { |
| 553 | std::lock_guard lock(interrupt_spin_lock); |
| 554 | NotThreadSafeCriticalSection(); |
| 555 | } |
| 556 | |
| 557 | |
| 558 | C |
| 559 | - |
| 560 | The InterruptSpinLock must be created in C++, however it can be passed into C using the |
| 561 | ``pw_sync_InterruptSpinLock`` opaque struct alias. |
| 562 | |
| 563 | .. cpp:function:: void pw_sync_InterruptSpinLock_Lock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 564 | |
| 565 | Invokes the ``InterruptSpinLock::lock`` member function on the given ``interrupt_spin_lock``. |
| 566 | |
| 567 | .. cpp:function:: bool pw_sync_InterruptSpinLock_TryLock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 568 | |
| 569 | Invokes the ``InterruptSpinLock::try_lock`` member function on the given ``interrupt_spin_lock``. |
| 570 | |
| 571 | .. cpp:function:: void pw_sync_InterruptSpinLock_Unlock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 572 | |
| 573 | Invokes the ``InterruptSpinLock::unlock`` member function on the given ``interrupt_spin_lock``. |
| 574 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 575 | .. list-table:: |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 576 | |
Ewout van Bekkum | fe70066 | 2021-04-02 16:48:50 -0700 | [diff] [blame] | 577 | * - *Safe to use in context* |
| 578 | - *Thread* |
| 579 | - *Interrupt* |
| 580 | - *NMI* |
| 581 | * - ``void pw_sync_InterruptSpinLock_Lock`` |
| 582 | - ✔ |
| 583 | - ✔ |
| 584 | - |
| 585 | * - ``bool pw_sync_InterruptSpinLock_TryLock`` |
| 586 | - ✔ |
| 587 | - ✔ |
| 588 | - |
| 589 | * - ``void pw_sync_InterruptSpinLock_Unlock`` |
| 590 | - ✔ |
| 591 | - ✔ |
| 592 | - |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 593 | |
| 594 | Example in C |
| 595 | ^^^^^^^^^^^^ |
| 596 | .. code-block:: cpp |
| 597 | |
| 598 | #include "pw_chrono/system_clock.h" |
| 599 | #include "pw_sync/interrupt_spin_lock.h" |
| 600 | |
| 601 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 602 | |
| 603 | extern pw_sync_InterruptSpinLock interrupt_spin_lock; // This can only be created in C++. |
| 604 | |
| 605 | void InterruptSafeCriticalSection(void) { |
| 606 | pw_sync_InterruptSpinLock_Lock(&interrupt_spin_lock); |
| 607 | NotThreadSafeCriticalSection(); |
| 608 | pw_sync_InterruptSpinLock_Unlock(&interrupt_spin_lock); |
| 609 | } |
| 610 | |
Ewout van Bekkum | cc9ef83 | 2021-04-08 08:51:16 -0700 | [diff] [blame] | 611 | Thread Safety Lock Annotations |
| 612 | ============================== |
| 613 | Pigweed's critical section lock primitives support Clang's thread safety |
| 614 | analysis extension for C++. The analysis is completely static at compile-time. |
| 615 | This is only supported when building with Clang. The annotations are no-ops when |
| 616 | using different compilers. |
| 617 | |
| 618 | Pigweed provides the ``pw_sync/lock_annotations.h`` header file with macro |
| 619 | definitions to allow developers to document the locking policies of |
| 620 | multi-threaded code. The annotations can also help program analysis tools to |
| 621 | identify potential thread safety issues. |
| 622 | |
| 623 | More information on Clang's thread safety analysis system can be found |
| 624 | `here <https://clang.llvm.org/docs/ThreadSafetyAnalysis.html>`_. |
| 625 | |
| 626 | Enabling Clang's Analysis |
| 627 | ------------------------- |
| 628 | In order to enable the analysis, Clang requires that the ``-Wthread-safety`` |
| 629 | compilation flag be used. In addition, if any STL components like |
| 630 | ``std::lock_guard`` are used, the STL's built in annotations have to be manually |
| 631 | enabled, typically by setting the ``_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS`` |
| 632 | macro. |
| 633 | |
| 634 | If using GN, the ``pw_build:clang_thread_safety_warnings`` config is provided |
| 635 | to do this for you, when added to your clang toolchain definition's default |
| 636 | configs. |
| 637 | |
| 638 | Why use lock annotations? |
| 639 | ------------------------- |
| 640 | Lock annotations can help warn you about potential race conditions in your code |
| 641 | when using locks: you have to remember to grab lock(s) before entering a |
| 642 | critical section, yuou have to remember to unlock it when you leave, and you |
| 643 | have to avoid deadlocks. |
| 644 | |
| 645 | Clang's lock annotations let you inform the compiler and anyone reading your |
| 646 | code which variables are guarded by which locks, which locks should or cannot be |
| 647 | held when calling which function, which order locks should be acquired in, etc. |
| 648 | |
| 649 | Using Lock Annotations |
| 650 | ---------------------- |
| 651 | When referring to locks in the arguments of the attributes, you should |
| 652 | use variable names or more complex expressions (e.g. ``my_object->lock_``) |
| 653 | that evaluate to a concrete lock object whenever possible. If the lock |
| 654 | you want to refer to is not in scope, you may use a member pointer |
| 655 | (e.g. ``&MyClass::lock_``) to refer to a lock in some (unknown) object. |
| 656 | |
| 657 | Annotating Lock Usage |
| 658 | ^^^^^^^^^^^^^^^^^^^^^ |
| 659 | .. cpp:function:: PW_GUARDED_BY(x) |
| 660 | |
| 661 | Documents if a shared field or global variable needs to be protected by a |
| 662 | lock. ``PW_GUARDED_BY()`` allows the user to specify a particular lock that |
| 663 | should be held when accessing the annotated variable. |
| 664 | |
| 665 | Although this annotation (and ``PW_PT_GUARDED_BY``, below) cannot be applied |
| 666 | to local variables, a local variable and its associated lock can often be |
| 667 | combined into a small class or struct, thereby allowing the annotation. |
| 668 | |
| 669 | Example: |
| 670 | |
| 671 | .. code-block:: cpp |
| 672 | |
| 673 | class Foo { |
| 674 | Mutex mu_; |
| 675 | int p1_ PW_GUARDED_BY(mu_); |
| 676 | ... |
| 677 | }; |
| 678 | |
| 679 | .. cpp:function:: PW_PT_GUARDED_BY(x) |
| 680 | |
| 681 | Documents if the memory location pointed to by a pointer should be guarded |
| 682 | by a lock when dereferencing the pointer. |
| 683 | |
| 684 | Example: |
| 685 | |
| 686 | .. code-block:: cpp |
| 687 | |
| 688 | class Foo { |
| 689 | Mutex mu_; |
| 690 | int *p1_ PW_PT_GUARDED_BY(mu_); |
| 691 | ... |
| 692 | }; |
| 693 | |
| 694 | Note that a pointer variable to a shared memory location could itself be a |
| 695 | shared variable. |
| 696 | |
| 697 | Example: |
| 698 | |
| 699 | .. code-block:: cpp |
| 700 | |
| 701 | // `q_`, guarded by `mu1_`, points to a shared memory location that is |
| 702 | // guarded by `mu2_`: |
| 703 | int *q_ PW_GUARDED_BY(mu1_) PW_PT_GUARDED_BY(mu2_); |
| 704 | |
| 705 | .. cpp:function:: PW_ACQUIRED_AFTER(...) |
| 706 | .. cpp:function:: PW_ACQUIRED_BEFORE(...) |
| 707 | |
| 708 | Documents the acquisition order between locks that can be held |
| 709 | simultaneously by a thread. For any two locks that need to be annotated |
| 710 | to establish an acquisition order, only one of them needs the annotation. |
| 711 | (i.e. You don't have to annotate both locks with both ``PW_ACQUIRED_AFTER`` |
| 712 | and ``PW_ACQUIRED_BEFORE``.) |
| 713 | |
| 714 | As with ``PW_GUARDED_BY``, this is only applicable to locks that are shared |
| 715 | fields or global variables. |
| 716 | |
| 717 | Example: |
| 718 | |
| 719 | .. code-block:: cpp |
| 720 | |
| 721 | Mutex m1_; |
| 722 | Mutex m2_ PW_ACQUIRED_AFTER(m1_); |
| 723 | |
| 724 | .. cpp:function:: PW_EXCLUSIVE_LOCKS_REQUIRED(...) |
| 725 | .. cpp:function:: PW_SHARED_LOCKS_REQUIRED(...) |
| 726 | |
| 727 | Documents a function that expects a lock to be held prior to entry. |
| 728 | The lock is expected to be held both on entry to, and exit from, the |
| 729 | function. |
| 730 | |
| 731 | An exclusive lock allows read-write access to the guarded data member(s), and |
| 732 | only one thread can acquire a lock exclusively at any one time. A shared lock |
| 733 | allows read-only access, and any number of threads can acquire a shared lock |
| 734 | concurrently. |
| 735 | |
| 736 | Generally, non-const methods should be annotated with |
| 737 | ``PW_EXCLUSIVE_LOCKS_REQUIRED``, while const methods should be annotated with |
| 738 | ``PW_SHARED_LOCKS_REQUIRED``. |
| 739 | |
| 740 | Example: |
| 741 | |
| 742 | .. code-block:: cpp |
| 743 | |
| 744 | Mutex mu1, mu2; |
| 745 | int a PW_GUARDED_BY(mu1); |
| 746 | int b PW_GUARDED_BY(mu2); |
| 747 | |
| 748 | void foo() PW_EXCLUSIVE_LOCKS_REQUIRED(mu1, mu2) { ... } |
| 749 | void bar() const PW_SHARED_LOCKS_REQUIRED(mu1, mu2) { ... } |
| 750 | |
| 751 | .. cpp:function:: PW_LOCKS_EXCLUDED(...) |
| 752 | |
| 753 | Documents the locks acquired in the body of the function. These locks |
| 754 | cannot be held when calling this function (as Pigweed's default locks are |
| 755 | non-reentrant). |
| 756 | |
| 757 | Example: |
| 758 | |
| 759 | .. code-block:: cpp |
| 760 | |
| 761 | Mutex mu; |
| 762 | int a PW_GUARDED_BY(mu); |
| 763 | |
| 764 | void foo() PW_LOCKS_EXCLUDED(mu) { |
| 765 | mu.lock(); |
| 766 | ... |
| 767 | mu.unlock(); |
| 768 | } |
| 769 | |
| 770 | .. cpp:function:: PW_LOCK_RETURNED(...) |
| 771 | |
| 772 | Documents a function that returns a lock without acquiring it. For example, |
| 773 | a public getter method that returns a pointer to a private lock should |
| 774 | be annotated with ``PW_LOCK_RETURNED``. |
| 775 | |
| 776 | Example: |
| 777 | |
| 778 | .. code-block:: cpp |
| 779 | |
| 780 | class Foo { |
| 781 | public: |
| 782 | Mutex* mu() PW_LOCK_RETURNED(mu) { return μ } |
| 783 | |
| 784 | private: |
| 785 | Mutex mu; |
| 786 | }; |
| 787 | |
| 788 | .. cpp:function:: PW_NO_LOCK_SAFETY_ANALYSIS() |
| 789 | |
| 790 | Turns off thread safety checking within the body of a particular function. |
| 791 | This annotation is used to mark functions that are known to be correct, but |
| 792 | the locking behavior is more complicated than the analyzer can handle. |
| 793 | |
| 794 | Annotating Lock Objects |
| 795 | ^^^^^^^^^^^^^^^^^^^^^^^ |
| 796 | In order of lock usage annotation to work, the lock objects themselves need to |
| 797 | be annotated as well. In case you are providing your own lock or psuedo-lock |
| 798 | object, you can use the macros in this section to annotate it. |
| 799 | |
| 800 | As an example we've annotated a Lock and a RAII ScopedLocker object for you, see |
| 801 | the macro documentation after for more details: |
| 802 | |
| 803 | .. code-block:: cpp |
| 804 | |
| 805 | class PW_LOCKABLE("Lock") Lock { |
| 806 | public: |
| 807 | void Lock() PW_EXCLUSIVE_LOCK_FUNCTION(); |
| 808 | |
| 809 | void ReaderLock() PW_SHARED_LOCK_FUNCTION(); |
| 810 | |
| 811 | void Unlock() PW_UNLOCK_FUNCTION(); |
| 812 | |
| 813 | void ReaderUnlock() PW_SHARED_TRYLOCK_FUNCTION(); |
| 814 | |
| 815 | bool TryLock() PW_EXCLUSIVE_TRYLOCK_FUNCTION(true); |
| 816 | |
| 817 | bool ReaderTryLock() PW_SHARED_TRYLOCK_FUNCTION(true); |
| 818 | |
| 819 | void AssertHeld() PW_ASSERT_EXCLUSIVE_LOCK(); |
| 820 | |
| 821 | void AssertReaderHeld() PW_ASSERT_SHARED_LOCK(); |
| 822 | }; |
| 823 | |
| 824 | |
| 825 | // Tag types for selecting a constructor. |
| 826 | struct adopt_lock_t {} inline constexpr adopt_lock = {}; |
| 827 | struct defer_lock_t {} inline constexpr defer_lock = {}; |
| 828 | struct shared_lock_t {} inline constexpr shared_lock = {}; |
| 829 | |
| 830 | class PW_SCOPED_LOCKABLE ScopedLocker { |
| 831 | // Acquire lock, implicitly acquire *this and associate it with lock. |
| 832 | ScopedLocker(Lock *lock) PW_EXCLUSIVE_LOCK_FUNCTION(lock) |
| 833 | : lock_(lock), locked(true) { |
| 834 | lock->Lock(); |
| 835 | } |
| 836 | |
| 837 | // Assume lock is held, implicitly acquire *this and associate it with lock. |
| 838 | ScopedLocker(Lock *lock, adopt_lock_t) PW_EXCLUSIVE_LOCKS_REQUIRED(lock) |
| 839 | : lock_(lock), locked(true) {} |
| 840 | |
| 841 | // Acquire lock in shared mode, implicitly acquire *this and associate it |
| 842 | // with lock. |
| 843 | ScopedLocker(Lock *lock, shared_lock_t) PW_SHARED_LOCK_FUNCTION(lock) |
| 844 | : lock_(lock), locked(true) { |
| 845 | lock->ReaderLock(); |
| 846 | } |
| 847 | |
| 848 | // Assume lock is held in shared mode, implicitly acquire *this and associate |
| 849 | // it with lock. |
| 850 | ScopedLocker(Lock *lock, adopt_lock_t, shared_lock_t) |
| 851 | PW_SHARED_LOCKS_REQUIRED(lock) : lock_(lock), locked(true) {} |
| 852 | |
| 853 | // Assume lock is not held, implicitly acquire *this and associate it with |
| 854 | // lock. |
| 855 | ScopedLocker(Lock *lock, defer_lock_t) PW_LOCKS_EXCLUDED(lock) |
| 856 | : lock_(lock), locked(false) {} |
| 857 | |
| 858 | // Release *this and all associated locks, if they are still held. |
| 859 | // There is no warning if the scope was already unlocked before. |
| 860 | ~ScopedLocker() PW_UNLOCK_FUNCTION() { |
| 861 | if (locked) |
| 862 | lock_->GenericUnlock(); |
| 863 | } |
| 864 | |
| 865 | // Acquire all associated locks exclusively. |
| 866 | void Lock() PW_EXCLUSIVE_LOCK_FUNCTION() { |
| 867 | lock_->Lock(); |
| 868 | locked = true; |
| 869 | } |
| 870 | |
| 871 | // Try to acquire all associated locks exclusively. |
| 872 | bool TryLock() PW_EXCLUSIVE_TRYLOCK_FUNCTION(true) { |
| 873 | return locked = lock_->TryLock(); |
| 874 | } |
| 875 | |
| 876 | // Acquire all associated locks in shared mode. |
| 877 | void ReaderLock() PW_SHARED_LOCK_FUNCTION() { |
| 878 | lock_->ReaderLock(); |
| 879 | locked = true; |
| 880 | } |
| 881 | |
| 882 | // Try to acquire all associated locks in shared mode. |
| 883 | bool ReaderTryLock() PW_SHARED_TRYLOCK_FUNCTION(true) { |
| 884 | return locked = lock_->ReaderTryLock(); |
| 885 | } |
| 886 | |
| 887 | // Release all associated locks. Warn on double unlock. |
| 888 | void Unlock() PW_UNLOCK_FUNCTION() { |
| 889 | lock_->Unlock(); |
| 890 | locked = false; |
| 891 | } |
| 892 | |
| 893 | // Release all associated locks. Warn on double unlock. |
| 894 | void ReaderUnlock() PW_UNLOCK_FUNCTION() { |
| 895 | lock_->ReaderUnlock(); |
| 896 | locked = false; |
| 897 | } |
| 898 | |
| 899 | private: |
| 900 | Lock* lock_; |
| 901 | bool locked_; |
| 902 | }; |
| 903 | |
| 904 | .. cpp:function:: PW_LOCKABLE(name) |
| 905 | |
| 906 | Documents if a class/type is a lockable type (such as the ``pw::sync::Mutex`` |
| 907 | class). The name is used in the warning messages. This can also be useful on |
| 908 | classes which have locking like semantics but aren't actually locks. |
| 909 | |
| 910 | .. cpp:function:: PW_SCOPED_LOCKABLE() |
| 911 | |
| 912 | Documents if a class does RAII locking. The name is used in the warning |
| 913 | messages. |
| 914 | |
| 915 | The constructor should use ``LOCK_FUNCTION()`` to specify the lock that is |
| 916 | acquired, and the destructor should use ``UNLOCK_FUNCTION()`` with no |
| 917 | arguments; the analysis will assume that the destructor unlocks whatever the |
| 918 | constructor locked. |
| 919 | |
| 920 | .. cpp:function:: PW_EXCLUSIVE_LOCK_FUNCTION() |
| 921 | |
| 922 | Documents functions that acquire a lock in the body of a function, and do |
| 923 | not release it. |
| 924 | |
| 925 | .. cpp:function:: PW_SHARED_LOCK_FUNCTION() |
| 926 | |
| 927 | Documents functions that acquire a shared (reader) lock in the body of a |
| 928 | function, and do not release it. |
| 929 | |
| 930 | .. cpp:function:: PW_UNLOCK_FUNCTION() |
| 931 | |
| 932 | Documents functions that expect a lock to be held on entry to the function, |
| 933 | and release it in the body of the function. |
| 934 | |
| 935 | .. cpp:function:: PW_EXCLUSIVE_TRYLOCK_FUNCTION(try_success) |
| 936 | .. cpp:function:: PW_SHARED_TRYLOCK_FUNCTION(try_success) |
| 937 | |
| 938 | Documents functions that try to acquire a lock, and return success or failure |
| 939 | (or a non-boolean value that can be interpreted as a boolean). |
| 940 | The first argument should be ``true`` for functions that return ``true`` on |
| 941 | success, or ``false`` for functions that return `false` on success. The second |
| 942 | argument specifies the lock that is locked on success. If unspecified, this |
| 943 | lock is assumed to be ``this``. |
| 944 | |
| 945 | .. cpp:function:: PW_ASSERT_EXCLUSIVE_LOCK() |
| 946 | .. cpp:function:: PW_ASSERT_SHARED_LOCK() |
| 947 | |
| 948 | Documents functions that dynamically check to see if a lock is held, and fail |
| 949 | if it is not held. |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 950 | |
| 951 | -------------------- |
| 952 | Signaling Primitives |
| 953 | -------------------- |
| 954 | |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 955 | Native signaling primitives tend to vary more compared to critial section locks |
| 956 | across different platforms. For example, although common signaling primtives |
| 957 | like semaphores are in most if not all RTOSes and even POSIX, it was not in the |
| 958 | STL before C++20. Likewise many C++ developers are surprised that conditional |
| 959 | variables tend to not be natively supported on RTOSes. Although you can usually |
| 960 | build any signaling primitive based on other native signaling primitives, this |
| 961 | may come with non-trivial added overhead in ROM, RAM, and execution efficiency. |
| 962 | |
Ewout van Bekkum | f010606 | 2021-05-06 14:08:33 -0700 | [diff] [blame] | 963 | For this reason, Pigweed intends to provide some simpler signaling primitives |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 964 | which exist to solve a narrow programming need but can be implemented as |
| 965 | efficiently as possible for the platform that it is used on. |
| 966 | |
| 967 | This simpler but highly portable class of signaling primitives is intended to |
| 968 | ensure that a portability efficiency tradeoff does not have to be made up front. |
Ewout van Bekkum | f010606 | 2021-05-06 14:08:33 -0700 | [diff] [blame] | 969 | Today this is class of simpler signaling primitives is limited to the |
| 970 | ``pw::sync::ThreadNotification`` and ``pw::sync::TimedThreadNotification``. |
| 971 | |
| 972 | ThreadNotification |
| 973 | ================== |
| 974 | The ThreadNotification is a synchronization primitive that can be used to |
| 975 | permit a SINGLE thread to block and consume a latching, saturating |
| 976 | notification from multiple notifiers. |
| 977 | |
Ewout van Bekkum | 0bf9625 | 2021-08-24 09:44:06 -0700 | [diff] [blame] | 978 | .. Note:: |
| 979 | Although only a single thread can block on a ThreadNotification at a time, |
| 980 | many instances may be used by a single thread just like binary semaphores. |
| 981 | This is in contrast to some native RTOS APIs, such as direct task |
| 982 | notifications, which re-use the same state within a thread's context. |
| 983 | |
Ewout van Bekkum | f010606 | 2021-05-06 14:08:33 -0700 | [diff] [blame] | 984 | .. Warning:: |
| 985 | This is a single consumer/waiter, multiple producer/notifier API! |
| 986 | The acquire APIs must only be invoked by a single consuming thread. As a |
| 987 | result, having multiple threads receiving notifications via the acquire API |
| 988 | is unsupported. |
| 989 | |
| 990 | This is effectively a subset of the ``pw::sync::BinarySemaphore`` API, except |
| 991 | that only a single thread can be notified and block at a time. |
| 992 | |
| 993 | The single consumer aspect of the API permits the use of a smaller and/or |
| 994 | faster native APIs such as direct thread signaling. This should be |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 995 | backed by the most efficient native primitive for a target, regardless of |
| 996 | whether that is a semaphore, event flag group, condition variable, or something |
| 997 | else. |
| 998 | |
Ewout van Bekkum | f010606 | 2021-05-06 14:08:33 -0700 | [diff] [blame] | 999 | Generic BinarySemaphore-based Backend |
| 1000 | ------------------------------------- |
| 1001 | This module provides a generic backend for ``pw::sync::ThreadNotification`` via |
| 1002 | ``pw_sync:binary_semaphore_thread_notification`` which uses a |
| 1003 | ``pw::sync::BinarySemaphore`` as the backing primitive. See |
| 1004 | :ref:`BinarySemaphore <module-pw_sync-binary-semaphore>` for backend |
| 1005 | availability. |
| 1006 | |
| 1007 | Optimized Backend |
| 1008 | ----------------- |
| 1009 | .. list-table:: |
| 1010 | |
| 1011 | * - *Supported on* |
| 1012 | - *Optimized backend module* |
| 1013 | * - FreeRTOS |
| 1014 | - Planned |
| 1015 | * - ThreadX |
| 1016 | - Planned |
| 1017 | * - embOS |
| 1018 | - Planned |
| 1019 | * - STL |
| 1020 | - Not planned, use ``pw_sync:binary_semaphore_thread_notification`` |
| 1021 | * - Baremetal |
| 1022 | - Planned |
| 1023 | * - Zephyr |
| 1024 | - Planned |
| 1025 | * - CMSIS-RTOS API v2 & RTX5 |
| 1026 | - Planned |
| 1027 | |
| 1028 | C++ |
| 1029 | --- |
| 1030 | .. cpp:class:: pw::sync::ThreadNotification |
| 1031 | |
| 1032 | .. cpp:function:: void acquire() |
| 1033 | |
| 1034 | Blocks indefinitely until the thread is notified, i.e. until the |
| 1035 | notification latch can be cleared because it was set. |
| 1036 | |
| 1037 | Clears the notification latch. |
| 1038 | |
| 1039 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1040 | |
| 1041 | .. cpp:function:: bool try_acquire() |
| 1042 | |
| 1043 | Returns whether the thread has been notified, i.e. whether the notificion |
| 1044 | latch was set and resets the latch regardless. |
| 1045 | |
| 1046 | Clears the notification latch. |
| 1047 | |
| 1048 | Returns true if the thread was notified, meaning the the internal latch was |
| 1049 | reset successfully. |
| 1050 | |
| 1051 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1052 | |
| 1053 | .. cpp:function:: void release() |
| 1054 | |
| 1055 | Notifies the thread in a saturating manner, setting the notification latch. |
| 1056 | |
| 1057 | Raising the notification multiple time without it being acquired by the |
| 1058 | consuming thread is equivalent to raising the notification once to the |
| 1059 | thread. The notification is latched in case the thread was not waiting at |
| 1060 | the time. |
| 1061 | |
| 1062 | This is IRQ and thread safe. |
| 1063 | |
| 1064 | .. list-table:: |
| 1065 | |
| 1066 | * - *Safe to use in context* |
| 1067 | - *Thread* |
| 1068 | - *Interrupt* |
| 1069 | - *NMI* |
| 1070 | * - ``ThreadNotification::ThreadNotification`` |
| 1071 | - ✔ |
| 1072 | - |
| 1073 | - |
| 1074 | * - ``ThreadNotification::~ThreadNotification`` |
| 1075 | - ✔ |
| 1076 | - |
| 1077 | - |
| 1078 | * - ``void ThreadNotification::acquire`` |
| 1079 | - ✔ |
| 1080 | - |
| 1081 | - |
| 1082 | * - ``bool ThreadNotification::try_acquire`` |
| 1083 | - ✔ |
| 1084 | - |
| 1085 | - |
| 1086 | * - ``void ThreadNotification::release`` |
| 1087 | - ✔ |
| 1088 | - ✔ |
| 1089 | - |
| 1090 | |
| 1091 | Examples in C++ |
| 1092 | ^^^^^^^^^^^^^^^ |
| 1093 | .. code-block:: cpp |
| 1094 | |
| 1095 | #include "pw_sync/thread_notification.h" |
| 1096 | #include "pw_thread/thread_core.h" |
| 1097 | |
| 1098 | class FooHandler() : public pw::thread::ThreadCore { |
| 1099 | // Public API invoked by other threads and/or interrupts. |
| 1100 | void NewFooAvailable() { |
| 1101 | new_foo_notification_.release(); |
| 1102 | } |
| 1103 | |
| 1104 | private: |
| 1105 | pw::sync::ThreadNotification new_foo_notification_; |
| 1106 | |
| 1107 | // Thread function. |
| 1108 | void Run() override { |
| 1109 | while (true) { |
| 1110 | new_foo_notification_.acquire(); |
| 1111 | HandleFoo(); |
| 1112 | } |
| 1113 | } |
| 1114 | |
| 1115 | void HandleFoo(); |
| 1116 | } |
| 1117 | |
| 1118 | TimedThreadNotification |
| 1119 | ======================= |
| 1120 | The TimedThreadNotification is an extension of the ThreadNotification which |
| 1121 | offers timeout and deadline based semantics. |
| 1122 | |
| 1123 | .. Warning:: |
| 1124 | This is a single consumer/waiter, multiple producer/notifier API! |
| 1125 | The acquire APIs must only be invoked by a single consuming thread. As a |
| 1126 | result, having multiple threads receiving notifications via the acquire API |
| 1127 | is unsupported. |
| 1128 | |
| 1129 | Generic BinarySemaphore-based Backend |
| 1130 | ------------------------------------- |
| 1131 | This module provides a generic backend for ``pw::sync::TimedThreadNotification`` |
| 1132 | via ``pw_sync:binary_semaphore_timed_thread_notification`` which uses a |
| 1133 | ``pw::sync::BinarySemaphore`` as the backing primitive. See |
| 1134 | :ref:`BinarySemaphore <module-pw_sync-binary-semaphore>` for backend |
| 1135 | availability. |
| 1136 | |
| 1137 | Optimized Backend |
| 1138 | ----------------- |
| 1139 | .. list-table:: |
| 1140 | |
| 1141 | * - *Supported on* |
| 1142 | - *Backend module* |
| 1143 | * - FreeRTOS |
| 1144 | - Planned |
| 1145 | * - ThreadX |
| 1146 | - Planned |
| 1147 | * - embOS |
| 1148 | - Planned |
| 1149 | * - STL |
| 1150 | - Not planned, use ``pw_sync:binary_semaphore_thread_notification`` |
| 1151 | * - Zephyr |
| 1152 | - Planned |
| 1153 | * - CMSIS-RTOS API v2 & RTX5 |
| 1154 | - Planned |
| 1155 | |
| 1156 | C++ |
| 1157 | --- |
| 1158 | .. cpp:class:: pw::sync::TimedThreadNotification |
| 1159 | |
| 1160 | .. cpp:function:: void acquire() |
| 1161 | |
| 1162 | Blocks indefinitely until the thread is notified, i.e. until the |
| 1163 | notification latch can be cleared because it was set. |
| 1164 | |
| 1165 | Clears the notification latch. |
| 1166 | |
| 1167 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1168 | |
| 1169 | .. cpp:function:: bool try_acquire() |
| 1170 | |
| 1171 | Returns whether the thread has been notified, i.e. whether the notificion |
| 1172 | latch was set and resets the latch regardless. |
| 1173 | |
| 1174 | Clears the notification latch. |
| 1175 | |
| 1176 | Returns true if the thread was notified, meaning the the internal latch was |
| 1177 | reset successfully. |
| 1178 | |
| 1179 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1180 | |
| 1181 | .. cpp:function:: void release() |
| 1182 | |
| 1183 | Notifies the thread in a saturating manner, setting the notification latch. |
| 1184 | |
| 1185 | Raising the notification multiple time without it being acquired by the |
| 1186 | consuming thread is equivalent to raising the notification once to the |
| 1187 | thread. The notification is latched in case the thread was not waiting at |
| 1188 | the time. |
| 1189 | |
| 1190 | This is IRQ and thread safe. |
| 1191 | |
| 1192 | .. cpp:function:: bool try_acquire_for(chrono::SystemClock::duration timeout) |
| 1193 | |
| 1194 | Blocks until the specified timeout duration has elapsed or the thread |
| 1195 | has been notified (i.e. notification latch can be cleared because it was |
| 1196 | set), whichever comes first. |
| 1197 | |
| 1198 | Clears the notification latch. |
| 1199 | |
| 1200 | Returns true if the thread was notified, meaning the the internal latch was |
| 1201 | reset successfully. |
| 1202 | |
| 1203 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1204 | |
| 1205 | .. cpp:function:: bool try_acquire_until(chrono::SystemClock::time_point deadline) |
| 1206 | |
| 1207 | Blocks until the specified deadline time has been reached the thread has |
| 1208 | been notified (i.e. notification latch can be cleared because it was set), |
| 1209 | whichever comes first. |
| 1210 | |
| 1211 | Clears the notification latch. |
| 1212 | |
| 1213 | Returns true if the thread was notified, meaning the the internal latch was |
| 1214 | reset successfully. |
| 1215 | |
| 1216 | **IMPORTANT:** This should only be used by a single consumer thread. |
| 1217 | |
| 1218 | .. list-table:: |
| 1219 | |
| 1220 | * - *Safe to use in context* |
| 1221 | - *Thread* |
| 1222 | - *Interrupt* |
| 1223 | - *NMI* |
| 1224 | * - ``ThreadNotification::ThreadNotification`` |
| 1225 | - ✔ |
| 1226 | - |
| 1227 | - |
| 1228 | * - ``ThreadNotification::~ThreadNotification`` |
| 1229 | - ✔ |
| 1230 | - |
| 1231 | - |
| 1232 | * - ``void ThreadNotification::acquire`` |
| 1233 | - ✔ |
| 1234 | - |
| 1235 | - |
| 1236 | * - ``bool ThreadNotification::try_acquire`` |
| 1237 | - ✔ |
| 1238 | - |
| 1239 | - |
| 1240 | * - ``bool ThreadNotification::try_acquire_for`` |
| 1241 | - ✔ |
| 1242 | - |
| 1243 | - |
| 1244 | * - ``bool ThreadNotification::try_acquire_until`` |
| 1245 | - ✔ |
| 1246 | - |
| 1247 | - |
| 1248 | * - ``void ThreadNotification::release`` |
| 1249 | - ✔ |
| 1250 | - ✔ |
| 1251 | - |
| 1252 | |
| 1253 | Examples in C++ |
| 1254 | ^^^^^^^^^^^^^^^ |
| 1255 | .. code-block:: cpp |
| 1256 | |
| 1257 | #include "pw_sync/timed_thread_notification.h" |
| 1258 | #include "pw_thread/thread_core.h" |
| 1259 | |
| 1260 | class FooHandler() : public pw::thread::ThreadCore { |
| 1261 | // Public API invoked by other threads and/or interrupts. |
| 1262 | void NewFooAvailable() { |
| 1263 | new_foo_notification_.release(); |
| 1264 | } |
| 1265 | |
| 1266 | private: |
| 1267 | pw::sync::TimedThreadNotification new_foo_notification_; |
| 1268 | |
| 1269 | // Thread function. |
| 1270 | void Run() override { |
| 1271 | while (true) { |
| 1272 | if (new_foo_notification_.try_acquire_for(kNotificationTimeout)) { |
| 1273 | HandleFoo(); |
| 1274 | } |
| 1275 | DoOtherStuff(); |
| 1276 | } |
| 1277 | } |
| 1278 | |
| 1279 | void HandleFoo(); |
| 1280 | void DoOtherStuff(); |
| 1281 | } |
| 1282 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 1283 | CountingSemaphore |
| 1284 | ================= |
| 1285 | The CountingSemaphore is a synchronization primitive that can be used for |
| 1286 | counting events and/or resource management where receiver(s) can block on |
| 1287 | acquire until notifier(s) signal by invoking release. |
| 1288 | |
| 1289 | Note that unlike Mutexes, priority inheritance is not used by semaphores meaning |
| 1290 | semaphores are subject to unbounded priority inversions. Due to this, Pigweed |
| 1291 | does not recommend semaphores for mutual exclusion. |
| 1292 | |
| 1293 | The CountingSemaphore is initialized to being empty or having no tokens. |
| 1294 | |
| 1295 | The entire API is thread safe, but only a subset is interrupt safe. None of it |
| 1296 | is NMI safe. |
| 1297 | |
| 1298 | .. Warning:: |
| 1299 | Releasing multiple tokens is often not natively supported, meaning you may |
| 1300 | end up invoking the native kernel API many times, i.e. once per token you |
| 1301 | are releasing! |
| 1302 | |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 1303 | .. list-table:: |
| 1304 | |
| 1305 | * - *Supported on* |
| 1306 | - *Backend module* |
| 1307 | * - FreeRTOS |
| 1308 | - :ref:`module-pw_sync_freertos` |
| 1309 | * - ThreadX |
| 1310 | - :ref:`module-pw_sync_threadx` |
| 1311 | * - embOS |
| 1312 | - :ref:`module-pw_sync_embos` |
| 1313 | * - STL |
| 1314 | - :ref:`module-pw_sync_stl` |
| 1315 | * - Zephyr |
| 1316 | - Planned |
| 1317 | * - CMSIS-RTOS API v2 & RTX5 |
| 1318 | - Planned |
| 1319 | |
Ewout van Bekkum | f010606 | 2021-05-06 14:08:33 -0700 | [diff] [blame] | 1320 | .. _module-pw_sync-binary-semaphore: |
| 1321 | |
Ewout van Bekkum | f84638b | 2021-03-12 16:09:08 -0800 | [diff] [blame] | 1322 | BinarySemaphore |
| 1323 | =============== |
| 1324 | BinarySemaphore is a specialization of CountingSemaphore with an arbitrary token |
| 1325 | limit of 1. Note that that ``max()`` is >= 1, meaning it may be released up to |
| 1326 | ``max()`` times but only acquired once for those N releases. |
| 1327 | |
| 1328 | Implementations of BinarySemaphore are typically more efficient than the |
| 1329 | default implementation of CountingSemaphore. |
| 1330 | |
| 1331 | The BinarySemaphore is initialized to being empty or having no tokens. |
| 1332 | |
| 1333 | The entire API is thread safe, but only a subset is interrupt safe. None of it |
| 1334 | is NMI safe. |
Ewout van Bekkum | 3b9eca4 | 2021-04-02 14:54:02 -0700 | [diff] [blame] | 1335 | |
| 1336 | .. list-table:: |
| 1337 | |
| 1338 | * - *Supported on* |
| 1339 | - *Backend module* |
| 1340 | * - FreeRTOS |
| 1341 | - :ref:`module-pw_sync_freertos` |
| 1342 | * - ThreadX |
| 1343 | - :ref:`module-pw_sync_threadx` |
| 1344 | * - embOS |
| 1345 | - :ref:`module-pw_sync_embos` |
| 1346 | * - STL |
| 1347 | - :ref:`module-pw_sync_stl` |
| 1348 | * - Zephyr |
| 1349 | - Planned |
| 1350 | * - CMSIS-RTOS API v2 & RTX5 |
| 1351 | - Planned |
| 1352 | |
Ewout van Bekkum | cc756c8 | 2021-05-12 07:57:43 -0700 | [diff] [blame] | 1353 | Conditional Variables |
| 1354 | ===================== |
| 1355 | We've decided for now to skip on conditional variables. These are constructs, |
| 1356 | which are typically not natively available on RTOSes. CVs would have to be |
| 1357 | backed by a multiple hidden semaphore(s) in addition to the explicit public |
| 1358 | mutex. In other words a CV typically ends up as a a composition of |
| 1359 | synchronization primitives on RTOSes. That being said, one could implement them |
| 1360 | using our semaphore and mutex layers and we may consider providing this in the |
| 1361 | future. However for most of our resource constrained customers they will mostly |
| 1362 | likely be using semaphores more often than CVs. |