.. _module-pw_sync_freertos:

================
pw_sync_freertos
================
This is a set of backends for pw_sync based on FreeRTOS.

--------------------------------
Critical Section Lock Primitives
--------------------------------

Mutex & TimedMutex
==================
The FreeRTOS backend for the Mutex and TimedMutex use ``StaticSemaphore_t`` as
the underlying type. It is created using ``xSemaphoreCreateMutexStatic`` as part
of the constructors and cleaned up using ``vSemaphoreDelete`` in the
destructors.

.. Note::
  Static allocation support is required in your FreeRTOS configuration, i.e.
  ``configSUPPORT_STATIC_ALLOCATION == 1``.

InterruptSpinLock
=================
The FreeRTOS backend for InterruptSpinLock is backed by ``UBaseType_t`` and a
``bool`` which permits these objects to stash the saved interrupt mask and to
detect accidental recursive locking.

This object uses ``taskENTER_CRITICAL_FROM_ISR`` and
``taskEXIT_CRITICAL_FROM_ISR`` from interrupt contexts, and
``taskENTER_CRITICAL`` and ``taskEXIT_CRITICAL`` in all other contexts.
``vTaskSuspendAll`` and ``xTaskResumeAll`` are additionally used within
lock/unlock respectively when called from task context in the scheduler-enabled
state.

.. Note::
  Scheduler State API support is required in your FreeRTOS Configuration, i.e.
  ``INCLUDE_xTaskGetSchedulerState == 1``.

.. warning::
  ``taskENTER_CRITICAL_FROM_ISR`` only disables interrupts with priority at or
  below ``configMAX_SYSCALL_INTERRUPT_PRIORITY``. Therefore, it is unsafe to
  use InterruptSpinLock from higher-priority interrupts, even if they are not
  non-maskable interrupts. This is consistent with the rest of the FreeRTOS
  APIs, see the `FreeRTOS kernel interrupt priority documentation
  <https://www.freertos.org/a00110.html#kernel_priority>`_ for more details.

Design Notes
------------
FreeRTOS does not supply an interrupt spin-lock API, so this backend provides
a suitable implementation using a compbination of both critical section and
schduler APIs provided by FreeRTOS.

This design is influenced by the following factors:

- FreeRTOS support for both synchronous and asynchronous yield behavior in
  different ports.
- Critical sections behave differently depending on whether or not yield is
  synchronous or asynchronous.
- Users must be allowed to call functions that result in a call to yield
  while an InterruptSpinLock is held.
- The signaling mechanisms in FreeRTOS all internally call yield to preempt
  the currently-running task in the event that a higher-priority task is
  unblocked during execution.

Synchronous and Asynchronous Yield
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In FreeRTOS, any kernel API call that results in a higher-priority task being
made “ready” triggers a call to ``taskYIELD()``.

In some ports, this results in an immediate context switch directly from
within the API - this is known as synchronous yielding behavior.

In other cases, this results in a software-triggered interrupt
being pended - and depending on the state of interrupts being masked, this
results in thread-scheduling being deferred until interrupts are unmasked.
This is known as asynchronous yielding behavior.

As part of a yield, it is left to the port-specific code to call
the FreeRTOS ``vTaskSwitchContext()`` function to swap current/ready tasks.
This function will select the next task to run, and swap it for the
currently executing task.

Yield Within a Critical Section
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A FreeRTOS critical section provides an interrupt-disabled context that ensures
that a thread of execution cannot be interrupted by incoming ISRs.

If a port implements asynchronous yield, any calls to ``taskYIELD()`` that
occur during execution of a critical section will not be handled until the
interrupts are re-enabled at the end of the critical section.  As a result,
any higher priority tasks that are unblocked will not preempt the current task
from within the critical section. In these ports, a critical section alone is
sufficient to prevent any interruption to code flow - be it from preempting
tasks or ISRs.

If a port implements synchronous yield, then a context switch to a
higher-priority ready task can occur within a critical section as a result
of a kernel API unblocking a higher-prirority task. When this occurs, the
higher-priority task will be swapped in immediately, and its interrupt-enabled
status applied to the CPU core. This typically causes interrupts to be
re-enabled as a result of the context switch, which is an unintended
side-effect for tasks that presume to have exclusive access to the CPU,
leading to logic errors and broken assumptions.

In short, any code that uses a FreeRTOS interrupt-disabled critical section
alone to provide an interrupt-safe context is subject to port-specific behavior
if it calls kernel APIs that can unblock tasks. A critical section alone is
insufficient to implement InterruptSpinLock correctly.

Yielding with Scheduling Suspended
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If a task is unblocked while the scheduler is suspended, the task is moved
to a "pending ready-list", and a flag is set to ensure that tasks are
scheduled as necessary once the scheduler is resumed.  Once scheduling
resumes, any tasks that were unblocked while the scheduler was suspended
are processed immediately, and rescheduling/preemption resumes at that time.

In the event that a call to ``taskYIELD()`` occurs directly while the
scheduler is suspended, the result is that ``vTaskSwitchContext()`` switches
back to the currently running task.  This is a guard-rail that short-circuits
any attempts to bypass the scheduler-suspended state manually.

Critical Section with Suspended Scheduling
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It is important to note that a critical section may be entered while the
scheduler is also disabled. In such a state, the system observes FreeRTOS'
contract that threads are not re-scheduled while the scheduler is supsended,
with the benefit that ISRs may not break the atomicity of code executing
while the lock is held.

This state is also compatible with either synchronous or asynchronous
yield behavior:

- In the synchronous cases, the result of a call to yield is that
  ``vTaskSwitchContext`` is invoked immediately, with the current task being
  restored.
- In the Asynchronous case, the result of a call to yield is that the context
  switch interrupt is deferred until the end of the critical section.

This is sufficient to satisfy the requirements implement an InterruptSpinLock
for any FreeRTOS target.

--------------------
Signaling Primitives
--------------------

ThreadNotification & TimedThreadNotification
============================================
An optimized FreeRTOS backend for the ThreadNotification and
TimedThreadNotification is provided using Task Notifications. It is backed by a
``TaskHandle_t`` and a ``bool`` which permits these objects to track the
notification value outside of the task's TCB (AKA FreeRTOS Task Notification
State and Value).

.. Warning::
  By default this backend uses the task notification at index 0, just like
  FreeRTOS Stream and Message Buffers. If you want to maintain the state of a
  task notification across blocking acquiring calls to ThreadNotifications, then
  you must do one of the following:

  1. Adjust ``PW_SYNC_FREERTOS_CONFIG_THREAD_NOTIFICATION_INDEX`` to an index
     which does not collide with existing incompatible use.
  2. Migrate your existing use of task notifications away from index 0.
  3. Do not use this optimized backend and instead use the binary semaphore
     backends for ThreadNotifications
     (``pw_sync:binary_semaphore_thread_notification_backend``).

  You are using any of the following Task Notification APIs, it means you are
  using notification indices:

  - ``xTaskNotify`` / ``xTaskNotifyIndexed``
  - ``xTaskNotifyFromISR`` / ``xTaskNotifyIndexedFromISR``
  - ``xTaskNotifyGive`` / ``xTaskNotifyGiveIndexed``
  - ``xTaskNotifyGiveFromISR`` / ``xTaskNotifyGiveIndexedFromISR``
  - ``xTaskNotifyAndQuery`` / ``xTaskNotifyAndQueryIndexed``
  - ``xTaskNotifyAndQueryFromISR`` / ``xTaskNotifyAndQueryIndexedFromISR``
  - ``ulTaskNotifyTake`` / ``ulTaskNotifyTakeIndexed``
  - ``xTaskNotifyWait`` / ``xTaskNotifyWaitIndexed``
  - ``xTaskNotifyStateClear`` / ``xTaskNotifyStateClearIndexed``
  - ``ulTaskNotifyValueClear`` / ``ulTaskNotifyValueClearIndexed``

  APIs without ``Indexed`` in the name use index 0 implicitly.

  Prior to FreeRTOS V10.4.0 each task had a single "notification index", and all
  task notification API functions operated on that implicit index of 0.

This backend is compatible with sharing the notification index
with native FreeRTOS
`Stream and Message Buffers <https://www.freertos.org/RTOS-task-notifications.html>`_
at index 0.

Just like FreeRTOS Stream and Message Buffers, this backend uses the task
notification index only within callsites where the task must block until a
notification is received or a timeout occurs. The notification index's state is
always cleaned up before returning. The notification index is never used when
the acquiring task is not going to block.

.. Note::
  Task notification support is required in your FreeRTOS configuration, i.e.
  ``configUSE_TASK_NOTIFICATIONS == 1``.

Design Notes
------------
You may ask, why are Task Notifications used at all given the risk associated
with global notification index allocations? It turns out there's no other
lightweight mechanism to unblock a task in FreeRTOS.

Task suspension (i.e. ``vTaskSuspend``, ``vTaskResume``, &
``vTaskResumeFromISR``) seems like a good fit, however ``xTaskResumeAll`` does
not participate in reference counting and will wake up all suspended tasks
whether you want it to or not.

Lastly, there's also ``xTaskAbortDelay`` but there is no interrupt safe
equivalent of this API. Note that it uses ``vTaskSuspendAll`` internally for
the critical section which is not interrupt safe. If in the future an interrupt
safe version of this API is offerred, then this would be a great alternative!

Lastly, we want to briefly explain how Task Notifications actually work in
FreeRTOS to show why you cannot directly share notification indeces even if the
bits used in the ``ulNotifiedValue`` are unique. This is a very common source of
bugs when using FreeRTOS and partially why Pigweed does not recommend using the
native Task Notification APIs directly.

FreeRTOS Task Notifications use a task's TCB's ``ucNotifyState`` to capture the
notification state even when the task is not blocked. This state transitions
``taskNOT_WAITING_NOTIFICATION`` to ``task_NOTIFICATION_RECEIVED`` if the task
ever notified. This notification state is used to determine whether the next
task notification wait call should block, irrespective of the notification
value.

In order to enable this optimized backend, native task notifications are only
used when the task needs to block. If a timeout occurs the task unregisters for
notifications and clears the notification state before returning. This exact
mechanism is used by FreeRTOS internally for their Stream and Message Buffer
implementations.

One other thing to note is that FreeRTOS has undocumented side effects between
``vTaskSuspend`` and ``xTaskNotifyWait``. If a thread is suspended via
``vTaskSuspend`` while blocked on ``xTaskNotifyWait``, the wait is aborted
regardless of the timeout (even if the request was indefinite) and the thread
is resumed whenever ``vTaskResume`` is invoked.

BinarySemaphore
===============
The FreeRTOS backend for the BinarySemaphore uses ``StaticSemaphore_t`` as the
underlying type. It is created using ``xSemaphoreCreateBinaryStatic`` as part
of the constructor and cleaned up using ``vSemaphoreDelete`` in the destructor.

.. Note::
  Static allocation support is required in your FreeRTOS configuration, i.e.
  ``configSUPPORT_STATIC_ALLOCATION == 1``.

CountingSemaphore
=================
The FreeRTOS backend for the CountingSemaphore uses ``StaticSemaphore_t`` as the
underlying type. It is created using ``xSemaphoreCreateCountingStatic`` as part
of the constructor and cleaned up using ``vSemaphoreDelete`` in the destructor.

.. Note::
  Counting semaphore support is required in your FreeRTOS configuration, i.e.
  ``configUSE_COUNTING_SEMAPHORES == 1``.
.. Note::
  Static allocation support is required in your FreeRTOS configuration, i.e.
  ``configSUPPORT_STATIC_ALLOCATION == 1``.