prvAcceptWaitClient gets interrupted with xSemaphoreTake while Tasks are suspended

Libraries
21

It depends on how the FreeRTOS_printf is implemented in your application. Its advised not to call any FreeRTOS API’s while the scheduler is suspended - vTaskSuspendAll

If FreeRTOS_printf is implemented to have FreeRTOS API calls, for example, sending the print text to a FreeRTOS queue that could block while the scheduler is suspended, then it can lead to undefined behaviors.

You can read a similar discussion here: Calling non-blocking FreeRTOS API functions while scheduler is suspended

22

Alright. I checked my implementation and yes. I did indeed use API functions within it.

	if( xLoggingEnter() != pdFALSE)
	{
		va_list xArgs;
		/* Watch out with small stacks. */
		char pcBuffer[ 128 ];
		BaseType_t xLength;

		va_start( xArgs, pcFormat );
		xLength = vsnprintf( pcBuffer, sizeof pcBuffer, pcFormat, xArgs );
		printf( pcBuffer, xLength );
		va_end( xArgs );
		vLoggingExit();
	}

and

static BaseType_t xLoggingEnter( void )
{
	BaseType_t xResult;

	if( xTaskGetSchedulerState() != taskSCHEDULER_RUNNING )  // <-- was == taskSCHEDULER_STARTED before 
	{
		xResult = pdFALSE;
	}
	else
	{
		if( xLoggingSemaphore == NULL )
		{
			/* Create a mutex. */
			xLoggingSemaphore = xSemaphoreCreateMutex();
		}

		if( xLoggingSemaphore != NULL )
		{
			/* Never wait longer than e.g. 2 ms. */
			TickType_t xTicksToWait = pdMS_TO_TICKS( 0 );

			xResult = xSemaphoreTake( xLoggingSemaphore, xTicksToWait );
		}
		else
		{
			xResult = pdFALSE;
		}
	}
	return xResult;
}

And while I understand the problem with calling api functions while the scheduler is suspended, I do need to use them in order to make my printf thread safe. Therefore the FreeRTOS_printf call in prvAcceptWaitClient should be removed regardless since it is either never called (as in my new version of printf) or not thread safe which might not be a problem here but can’t be good practice in a OS.

23

Yes, it should be removed. We have created a pull request, which is under review, addressing the issue here.

Thanks

2 Likes