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.


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


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.


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


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 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).


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 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.


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.


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.


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


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.


Counting semaphore support is required in your FreeRTOS configuration, i.e. configUSE_COUNTING_SEMAPHORES == 1.


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