pw_sync_threadx

This is a set of backends for pw_sync based on ThreadX.

It is possible, if necessary, to use pw_sync_threadx without using the Pigweed provided pw_chrono_threadx in case the ThreadX time API (tx_time_get())) is not available (i.e. TX_NO_TIMER is set). You are responsible for ensuring that the chrono backend provided has counts which match the ThreadX tick based API.

Critical Section Lock Primitives

Mutex & TimedMutex

The ThreadX backend for the Mutex and TimedMutex use TX_MUTEX as the underlying type. It is created using tx_mutex_create as part of the constructors and cleaned up using tx_mutex_delete in the destructors.

InterruptSpinLock

The ThreadX backend for InterruptSpinLock is backed by an enum class and two UINT which permits these objects to detect accidental recursive locking and unlocking contexts.

This object uses tx_interrupt_control to enable critical sections. In addition, tx_thread_preemption_change is used to prevent accidental thread context switches while the InterruptSpinLock is held by a thread.

Warning

This backend does not support SMP yet as there’s no internal lock to spin on.

Signaling Primitives

ThreadNotification & TimedThreadNotification

Prefer using the binary semaphore backends for ThreadNotifications as the native ThreadX API covers direct thread signaling:

• pw_sync:binary_semaphore_thread_notification_backend

• pw_sync:binary_semaphore_timed_thread_notification_backend

Background Information

Although one may be tempted to use tx_thread_sleep and tx_thread_wait_abort to implement direct thread notifications with ThreadX, this unfortunately cannot work. Between the blocking thread setting its TX_THREAD* handle and actually executing tx_thread_sleep there will always exist a race condition. Another thread and/or interrupt may attempt to invoke tx_thread_wait_abort before the blocking thread has executed tx_thread_sleep meaning the wait abort would fail.

BinarySemaphore & CountingSemaphore

The ThreadX backends for the BinarySemaphore and CountingSemaphore use TX_SEMAPHORE as the underlying type. It is created using tx_semaphore_create as part of the constructor and cleaned up using tx_semaphore_delete in the destructor.