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 | .. contents:: |
| 11 | :local: |
| 12 | :depth: 2 |
| 13 | |
| 14 | .. Warning:: |
| 15 | This module is still under construction, the API is not yet stable. |
| 16 | |
| 17 | .. Note:: |
| 18 | The objects in this module do not have an Init() style public API which is |
| 19 | common in many RTOS C APIs. Instead, they rely on being able to invoke the |
| 20 | native initialization APIs for synchronization primitives during C++ |
| 21 | construction. |
| 22 | In order to support global statically constructed synchronization without |
| 23 | constexpr constructors, the user and/or backend **MUST** ensure that any |
| 24 | initialization required in your environment is done prior to the creation |
| 25 | and/or initialization of the native synchronization primitives |
| 26 | (e.g. kernel initialization). |
| 27 | |
| 28 | -------------------------------- |
| 29 | Critical Section Lock Primitives |
| 30 | -------------------------------- |
| 31 | The critical section lock primitives provided by this module comply with |
| 32 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_, |
| 33 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_, and where |
| 34 | relevant |
| 35 | `TimedLockable <https://en.cppreference.com/w/cpp/named_req/TimedLockable>`_ C++ |
| 36 | named requirements. This means that they are compatible with existing helpers in |
| 37 | the STL's ``<mutex>`` thread support library. For example `std::lock_guard <https://en.cppreference.com/w/cpp/thread/lock_guard>`_ |
| 38 | and `std::unique_lock <https://en.cppreference.com/w/cpp/thread/unique_lock>`_ can be directly used. |
| 39 | |
| 40 | Mutex |
| 41 | ===== |
| 42 | The Mutex is a synchronization primitive that can be used to protect shared data |
| 43 | from being simultaneously accessed by multiple threads. It offers exclusive, |
| 44 | non-recursive ownership semantics where priority inheritance is used to solve |
| 45 | the classic priority-inversion problem. |
| 46 | |
| 47 | The Mutex's API is C++11 STL |
| 48 | `std::timed_mutex <https://en.cppreference.com/w/cpp/thread/timed_mutex>`_ like, |
| 49 | meaning it is a |
| 50 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_, |
| 51 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_, and |
| 52 | `TimedLockable <https://en.cppreference.com/w/cpp/named_req/TimedLockable>`_. |
| 53 | |
| 54 | |
| 55 | C++ |
| 56 | --- |
| 57 | .. cpp:class:: pw::sync::Mutex |
| 58 | |
| 59 | .. cpp:function:: void lock() |
| 60 | |
| 61 | Locks the mutex, blocking indefinitely. Failures are fatal. |
| 62 | |
| 63 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 64 | locking is undefined behavior. |
| 65 | |
| 66 | .. cpp:function:: bool try_lock() |
| 67 | |
| 68 | Attempts to lock the mutex in a non-blocking manner. |
| 69 | Returns true if the mutex was successfully acquired. |
| 70 | |
| 71 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 72 | locking is undefined behavior. |
| 73 | |
| 74 | .. cpp:function:: bool try_lock_for(chrono::SystemClock::duration for_at_least) |
| 75 | |
| 76 | Attempts to lock the mutex where, if needed, blocking for at least the |
| 77 | specified duration. |
| 78 | Returns true if the mutex was successfully acquired. |
| 79 | |
| 80 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 81 | locking is undefined behavior. |
| 82 | |
| 83 | .. cpp:function:: bool try_lock_until(chrono::SystemClock::time_point until_at_least) |
| 84 | |
| 85 | Attempts to lock the mutex where, if needed, blocking until at least the |
| 86 | specified time_point. |
| 87 | Returns true if the mutex was successfully acquired. |
| 88 | |
| 89 | **Precondition:** The lock isn't already held by this thread. Recursive |
| 90 | locking is undefined behavior. |
| 91 | |
| 92 | .. cpp:function:: void unlock() |
| 93 | |
| 94 | Unlocks the mutex. Failures are fatal. |
| 95 | |
| 96 | **Precondition:** The mutex is held by this thread. |
| 97 | |
| 98 | +--------------------------------+----------+-------------+-------+ |
| 99 | | *Safe to use in context* | *Thread* | *Interrupt* | *NMI* | |
| 100 | +--------------------------------+----------+-------------+-------+ |
| 101 | | ``Mutex::Mutex`` | ✔ | | | |
| 102 | +--------------------------------+----------+-------------+-------+ |
| 103 | | ``Mutex::~Mutex`` | ✔ | | | |
| 104 | +--------------------------------+----------+-------------+-------+ |
| 105 | | ``void Mutex::lock`` | ✔ | | | |
| 106 | +--------------------------------+----------+-------------+-------+ |
| 107 | | ``bool Mutex::try_lock`` | ✔ | | | |
| 108 | +--------------------------------+----------+-------------+-------+ |
| 109 | | ``bool Mutex::try_lock_for`` | ✔ | | | |
| 110 | +--------------------------------+----------+-------------+-------+ |
| 111 | | ``bool Mutex::try_lock_until`` | ✔ | | | |
| 112 | +--------------------------------+----------+-------------+-------+ |
| 113 | | ``void Mutex::unlock`` | ✔ | | | |
| 114 | +--------------------------------+----------+-------------+-------+ |
| 115 | |
| 116 | |
| 117 | Examples in C++ |
| 118 | ^^^^^^^^^^^^^^^ |
| 119 | .. code-block:: cpp |
| 120 | |
| 121 | #include "pw_chrono/system_clock.h" |
| 122 | #include "pw_sync/mutex.h" |
| 123 | |
| 124 | pw::sync::Mutex mutex; |
| 125 | |
| 126 | void ThreadSafeCriticalSection() { |
| 127 | mutex.lock(); |
| 128 | NotThreadSafeCriticalSection(); |
| 129 | mutex.unlock(); |
| 130 | } |
| 131 | |
| 132 | bool ThreadSafeCriticalSectionWithTimeout( |
| 133 | const SystemClock::duration timeout) { |
| 134 | if (!mutex.try_lock_for(timeout)) { |
| 135 | return false; |
| 136 | } |
| 137 | NotThreadSafeCriticalSection(); |
| 138 | mutex.unlock(); |
| 139 | return true; |
| 140 | } |
| 141 | |
| 142 | |
| 143 | Alternatively you can use C++'s RAII helpers to ensure you always unlock. |
| 144 | |
| 145 | .. code-block:: cpp |
| 146 | |
| 147 | #include <mutex> |
| 148 | |
| 149 | #include "pw_chrono/system_clock.h" |
| 150 | #include "pw_sync/mutex.h" |
| 151 | |
| 152 | pw::sync::Mutex mutex; |
| 153 | |
| 154 | void ThreadSafeCriticalSection() { |
| 155 | std::lock_guard lock(mutex); |
| 156 | NotThreadSafeCriticalSection(); |
| 157 | } |
| 158 | |
| 159 | bool ThreadSafeCriticalSectionWithTimeout( |
| 160 | const SystemClock::duration timeout) { |
| 161 | std::unique_lock lock(mutex, std::defer_lock); |
| 162 | if (!lock.try_lock_for(timeout)) { |
| 163 | return false; |
| 164 | } |
| 165 | NotThreadSafeCriticalSection(); |
| 166 | return true; |
| 167 | } |
| 168 | |
| 169 | |
| 170 | |
| 171 | C |
| 172 | - |
| 173 | The Mutex must be created in C++, however it can be passed into C using the |
| 174 | ``pw_sync_Mutex`` opaque struct alias. |
| 175 | |
| 176 | .. cpp:function:: void pw_sync_Mutex_Lock(pw_sync_Mutex* mutex) |
| 177 | |
| 178 | Invokes the ``Mutex::lock`` member function on the given ``mutex``. |
| 179 | |
| 180 | .. cpp:function:: bool pw_sync_Mutex_TryLock(pw_sync_Mutex* mutex) |
| 181 | |
| 182 | Invokes the ``Mutex::try_lock`` member function on the given ``mutex``. |
| 183 | |
| 184 | .. cpp:function:: bool pw_sync_Mutex_TryLockFor(pw_sync_Mutex* mutex, pw_chrono_SystemClock_Duration for_at_least) |
| 185 | |
| 186 | Invokes the ``Mutex::try_lock_for`` member function on the given ``mutex``. |
| 187 | |
| 188 | .. cpp:function:: bool pw_sync_Mutex_TryLockUntil(pw_sync_Mutex* mutex, pw_chrono_SystemClock_TimePoint until_at_least) |
| 189 | |
| 190 | Invokes the ``Mutex::try_lock_until`` member function on the given ``mutex``. |
| 191 | |
| 192 | .. cpp:function:: void pw_sync_Mutex_Unlock(pw_sync_Mutex* mutex) |
| 193 | |
| 194 | Invokes the ``Mutex::unlock`` member function on the given ``mutex``. |
| 195 | |
| 196 | +-------------------------------------+----------+-------------+-------+ |
| 197 | | *Safe to use in context* | *Thread* | *Interrupt* | *NMI* | |
| 198 | +-------------------------------------+----------+-------------+-------+ |
| 199 | | ``void pw_sync_Mutex_Lock`` | ✔ | | | |
| 200 | +-------------------------------------+----------+-------------+-------+ |
| 201 | | ``bool pw_sync_Mutex_TryLock`` | ✔ | | | |
| 202 | +-------------------------------------+----------+-------------+-------+ |
| 203 | | ``bool pw_sync_Mutex_TryLockFor`` | ✔ | | | |
| 204 | +-------------------------------------+----------+-------------+-------+ |
| 205 | | ``bool pw_sync_Mutex_TryLockUntil`` | ✔ | | | |
| 206 | +-------------------------------------+----------+-------------+-------+ |
| 207 | | ``void pw_sync_Mutex_Unlock`` | ✔ | | | |
| 208 | +-------------------------------------+----------+-------------+-------+ |
| 209 | |
| 210 | |
| 211 | Example in C |
| 212 | ^^^^^^^^^^^^ |
| 213 | .. code-block:: cpp |
| 214 | |
| 215 | #include "pw_chrono/system_clock.h" |
| 216 | #include "pw_sync/mutex.h" |
| 217 | |
| 218 | pw::sync::Mutex mutex; |
| 219 | |
| 220 | extern pw_sync_Mutex mutex; // This can only be created in C++. |
| 221 | |
| 222 | void ThreadSafeCriticalSection(void) { |
| 223 | pw_sync_Mutex_Lock(&mutex); |
| 224 | NotThreadSafeCriticalSection(); |
| 225 | pw_sync_Mutex_Unlock(&mutex); |
| 226 | } |
| 227 | |
| 228 | bool ThreadSafeCriticalSectionWithTimeout( |
| 229 | const pw_chrono_SystemClock_Duration timeout) { |
| 230 | if (!pw_sync_Mutex_TryLockFor(&mutex, timeout)) { |
| 231 | return false; |
| 232 | } |
| 233 | NotThreadSafeCriticalSection(); |
| 234 | pw_sync_Mutex_Unlock(&mutex); |
| 235 | return true; |
| 236 | } |
| 237 | |
| 238 | |
| 239 | InterruptSpinLock |
| 240 | ================= |
| 241 | The InterruptSpinLock is a synchronization primitive that can be used to protect |
| 242 | shared data from being simultaneously accessed by multiple threads and/or |
| 243 | interrupts as a targeted global lock, with the exception of Non-Maskable |
| 244 | Interrupts (NMIs). It offers exclusive, non-recursive ownership semantics where |
| 245 | IRQs up to a backend defined level of "NMIs" will be masked to solve |
| 246 | priority-inversion. |
| 247 | |
| 248 | This InterruptSpinLock relies on built-in local interrupt masking to make it |
| 249 | interrupt safe without requiring the caller to separately mask and unmask |
| 250 | interrupts when using this primitive. |
| 251 | |
| 252 | Unlike global interrupt locks, this also works safely and efficiently on SMP |
| 253 | systems. On systems which are not SMP, spinning is not required but some state |
| 254 | may still be used to detect recursion. |
| 255 | |
| 256 | The InterruptSpinLock is a |
| 257 | `BasicLockable <https://en.cppreference.com/w/cpp/named_req/BasicLockable>`_ |
| 258 | and |
| 259 | `Lockable <https://en.cppreference.com/w/cpp/named_req/Lockable>`_. |
| 260 | |
| 261 | |
| 262 | C++ |
| 263 | --- |
| 264 | .. cpp:class:: pw::sync::InterruptSpinLock |
| 265 | |
| 266 | .. cpp:function:: void lock() |
| 267 | |
| 268 | Locks the spinlock, blocking indefinitely. Failures are fatal. |
| 269 | |
| 270 | **Precondition:** Recursive locking is undefined behavior. |
| 271 | |
| 272 | .. cpp:function:: bool try_lock() |
| 273 | |
| 274 | Attempts to lock the spinlock in a non-blocking manner. |
| 275 | Returns true if the spinlock was successfully acquired. |
| 276 | |
| 277 | **Precondition:** Recursive locking is undefined behavior. |
| 278 | |
| 279 | .. cpp:function:: void unlock() |
| 280 | |
| 281 | Unlocks the mutex. Failures are fatal. |
| 282 | |
| 283 | **Precondition:** The spinlock is held by the caller. |
| 284 | |
| 285 | +-------------------------------------------+----------+-------------+-------+ |
| 286 | | *Safe to use in context* | *Thread* | *Interrupt* | *NMI* | |
| 287 | +-------------------------------------------+----------+-------------+-------+ |
| 288 | | ``InterruptSpinLock::InterruptSpinLock`` | ✔ | ✔ | | |
| 289 | +-------------------------------------------+----------+-------------+-------+ |
| 290 | | ``InterruptSpinLock::~InterruptSpinLock`` | ✔ | ✔ | | |
| 291 | +-------------------------------------------+----------+-------------+-------+ |
| 292 | | ``void InterruptSpinLock::lock`` | ✔ | ✔ | | |
| 293 | +-------------------------------------------+----------+-------------+-------+ |
| 294 | | ``bool InterruptSpinLock::try_lock`` | ✔ | ✔ | | |
| 295 | +-------------------------------------------+----------+-------------+-------+ |
| 296 | | ``void InterruptSpinLock::unlock`` | ✔ | ✔ | | |
| 297 | +-------------------------------------------+----------+-------------+-------+ |
| 298 | |
| 299 | |
| 300 | Examples in C++ |
| 301 | ^^^^^^^^^^^^^^^ |
| 302 | .. code-block:: cpp |
| 303 | |
| 304 | #include "pw_sync/interrupt_spin_lock.h" |
| 305 | |
| 306 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 307 | |
| 308 | void InterruptSafeCriticalSection() { |
| 309 | interrupt_spin_lock.lock(); |
| 310 | NotThreadSafeCriticalSection(); |
| 311 | interrupt_spin_lock.unlock(); |
| 312 | } |
| 313 | |
| 314 | |
| 315 | Alternatively you can use C++'s RAII helpers to ensure you always unlock. |
| 316 | |
| 317 | .. code-block:: cpp |
| 318 | |
| 319 | #include <mutex> |
| 320 | |
| 321 | #include "pw_sync/interrupt_spin_lock.h" |
| 322 | |
| 323 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 324 | |
| 325 | void InterruptSafeCriticalSection() { |
| 326 | std::lock_guard lock(interrupt_spin_lock); |
| 327 | NotThreadSafeCriticalSection(); |
| 328 | } |
| 329 | |
| 330 | |
| 331 | C |
| 332 | - |
| 333 | The InterruptSpinLock must be created in C++, however it can be passed into C using the |
| 334 | ``pw_sync_InterruptSpinLock`` opaque struct alias. |
| 335 | |
| 336 | .. cpp:function:: void pw_sync_InterruptSpinLock_Lock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 337 | |
| 338 | Invokes the ``InterruptSpinLock::lock`` member function on the given ``interrupt_spin_lock``. |
| 339 | |
| 340 | .. cpp:function:: bool pw_sync_InterruptSpinLock_TryLock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 341 | |
| 342 | Invokes the ``InterruptSpinLock::try_lock`` member function on the given ``interrupt_spin_lock``. |
| 343 | |
| 344 | .. cpp:function:: void pw_sync_InterruptSpinLock_Unlock(pw_sync_InterruptSpinLock* interrupt_spin_lock) |
| 345 | |
| 346 | Invokes the ``InterruptSpinLock::unlock`` member function on the given ``interrupt_spin_lock``. |
| 347 | |
| 348 | +--------------------------------------------+----------+-------------+-------+ |
| 349 | | *Safe to use in context* | *Thread* | *Interrupt* | *NMI* | |
| 350 | +--------------------------------------------+----------+-------------+-------+ |
| 351 | | ``void pw_sync_InterruptSpinLock_Lock`` | ✔ | ✔ | | |
| 352 | +--------------------------------------------+----------+-------------+-------+ |
| 353 | | ``bool pw_sync_InterruptSpinLock_TryLock`` | ✔ | ✔ | | |
| 354 | +--------------------------------------------+----------+-------------+-------+ |
| 355 | | ``void pw_sync_InterruptSpinLock_Unlock`` | ✔ | ✔ | | |
| 356 | +--------------------------------------------+----------+-------------+-------+ |
| 357 | |
| 358 | |
| 359 | Example in C |
| 360 | ^^^^^^^^^^^^ |
| 361 | .. code-block:: cpp |
| 362 | |
| 363 | #include "pw_chrono/system_clock.h" |
| 364 | #include "pw_sync/interrupt_spin_lock.h" |
| 365 | |
| 366 | pw::sync::InterruptSpinLock interrupt_spin_lock; |
| 367 | |
| 368 | extern pw_sync_InterruptSpinLock interrupt_spin_lock; // This can only be created in C++. |
| 369 | |
| 370 | void InterruptSafeCriticalSection(void) { |
| 371 | pw_sync_InterruptSpinLock_Lock(&interrupt_spin_lock); |
| 372 | NotThreadSafeCriticalSection(); |
| 373 | pw_sync_InterruptSpinLock_Unlock(&interrupt_spin_lock); |
| 374 | } |
| 375 | |
| 376 | |
| 377 | -------------------- |
| 378 | Signaling Primitives |
| 379 | -------------------- |
| 380 | |
| 381 | CountingSemaphore |
| 382 | ================= |
| 383 | The CountingSemaphore is a synchronization primitive that can be used for |
| 384 | counting events and/or resource management where receiver(s) can block on |
| 385 | acquire until notifier(s) signal by invoking release. |
| 386 | |
| 387 | Note that unlike Mutexes, priority inheritance is not used by semaphores meaning |
| 388 | semaphores are subject to unbounded priority inversions. Due to this, Pigweed |
| 389 | does not recommend semaphores for mutual exclusion. |
| 390 | |
| 391 | The CountingSemaphore is initialized to being empty or having no tokens. |
| 392 | |
| 393 | The entire API is thread safe, but only a subset is interrupt safe. None of it |
| 394 | is NMI safe. |
| 395 | |
| 396 | .. Warning:: |
| 397 | Releasing multiple tokens is often not natively supported, meaning you may |
| 398 | end up invoking the native kernel API many times, i.e. once per token you |
| 399 | are releasing! |
| 400 | |
| 401 | BinarySemaphore |
| 402 | =============== |
| 403 | BinarySemaphore is a specialization of CountingSemaphore with an arbitrary token |
| 404 | limit of 1. Note that that ``max()`` is >= 1, meaning it may be released up to |
| 405 | ``max()`` times but only acquired once for those N releases. |
| 406 | |
| 407 | Implementations of BinarySemaphore are typically more efficient than the |
| 408 | default implementation of CountingSemaphore. |
| 409 | |
| 410 | The BinarySemaphore is initialized to being empty or having no tokens. |
| 411 | |
| 412 | The entire API is thread safe, but only a subset is interrupt safe. None of it |
| 413 | is NMI safe. |