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Fix xTaskNotifyWait & ulTaskNotifyTake determinism. #625

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Fix xTaskNotifyWait & ulTaskNotifyTake determinism. #625

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8daadaf
Fix task notification determinism.
karver8 Feb 13, 2023
293fbcd
Merge branch 'main' into deterministic-task-notify
n9wxu May 5, 2023
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80 changes: 46 additions & 34 deletions tasks.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3137,8 +3137,8 @@ void vTaskPlaceOnEventList( List_t * const pxEventList,
{
configASSERT( pxEventList );

/* THIS FUNCTION MUST BE CALLED WITH EITHER INTERRUPTS DISABLED OR THE
* SCHEDULER SUSPENDED AND THE QUEUE BEING ACCESSED LOCKED. */
/* THIS FUNCTION MUST BE CALLED WITH THE SCHEDULER SUSPENDED AND THE QUEUE
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Why was this comment changed? The original "Interrupts disabled" should probably be "inside a critical section", but I think it's still valid.

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I changed it because this function calls vListInsert(), which shouldn't be called while interrupts are disabled as far as I am aware. I noted this comment in my original issue. But I can change it back if you think this is not correct.

* BEING ACCESSED LOCKED. */

/* Place the event list item of the TCB in the appropriate event list.
* This is placed in the list in priority order so the highest priority task
Expand Down Expand Up @@ -4744,22 +4744,25 @@ TickType_t uxTaskResetEventItemValue( void )
configASSERT( uxIndexToWait < configTASK_NOTIFICATION_ARRAY_ENTRIES );

taskENTER_CRITICAL();

/* Only block if the notification count is not already non-zero. */
if( pxCurrentTCB->ulNotifiedValue[ uxIndexToWait ] == 0UL )
{
/* Only block if the notification count is not already non-zero. */
if( pxCurrentTCB->ulNotifiedValue[ uxIndexToWait ] == 0UL )
/* Mark this task as waiting for a notification. */
pxCurrentTCB->ucNotifyState[ uxIndexToWait ] = taskWAITING_NOTIFICATION;

if( xTicksToWait > ( TickType_t ) 0 )
{
/* Mark this task as waiting for a notification. */
pxCurrentTCB->ucNotifyState[ uxIndexToWait ] = taskWAITING_NOTIFICATION;
traceTASK_NOTIFY_TAKE_BLOCK( uxIndexToWait );

if( xTicksToWait > ( TickType_t ) 0 )
{
prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );
traceTASK_NOTIFY_TAKE_BLOCK( uxIndexToWait );
/* Suspend the scheduler before enabling interrupts. */
vTaskSuspendAll();
taskEXIT_CRITICAL();
Comment on lines +4758 to +4760
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Strictly speaking, the API doesn't allow calling vTaskSuspendAll() from inside a critical section. It probably works, but it would be the only place it happens in the FreeRTOS codebase.

The intent behind the larger construct being proposed here seems to be to avoid suspending/resuming the scheduler if possible, which in turn keeps ulTaskGenericNotifyTake() as lightweight as possible. Maybe a different construct could be used instead. For example, the idle task uses a different construct to avoid suspending/resuming the scheduler for tickless idle. A similar approach might work here -- specifically, checking the notification value while "unprotected" to determine whether to suspend the scheduler, and then verifying the notification value after suspending the scheduler if suspending is warranted.


prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );

/* All ports are written to allow a yield in a critical
* section (some will yield immediately, others wait until the
* critical section exits) - but it is not something that
* application code should ever do. */
if( xTaskResumeAll() == pdFALSE )
{
portYIELD_WITHIN_API();
}
else
Expand All @@ -4769,10 +4772,13 @@ TickType_t uxTaskResetEventItemValue( void )
}
else
{
mtCOVERAGE_TEST_MARKER();
taskEXIT_CRITICAL();
}
}
taskEXIT_CRITICAL();
else
{
taskEXIT_CRITICAL();
}

taskENTER_CRITICAL();
{
Expand Down Expand Up @@ -4818,27 +4824,30 @@ TickType_t uxTaskResetEventItemValue( void )
configASSERT( uxIndexToWait < configTASK_NOTIFICATION_ARRAY_ENTRIES );

taskENTER_CRITICAL();

/* Only block if a notification is not already pending. */
if( pxCurrentTCB->ucNotifyState[ uxIndexToWait ] != taskNOTIFICATION_RECEIVED )
{
/* Only block if a notification is not already pending. */
if( pxCurrentTCB->ucNotifyState[ uxIndexToWait ] != taskNOTIFICATION_RECEIVED )
/* Clear bits in the task's notification value as bits may get
* set by the notifying task or interrupt. This can be used to
* clear the value to zero. */
pxCurrentTCB->ulNotifiedValue[ uxIndexToWait ] &= ~ulBitsToClearOnEntry;

/* Mark this task as waiting for a notification. */
pxCurrentTCB->ucNotifyState[ uxIndexToWait ] = taskWAITING_NOTIFICATION;

if( xTicksToWait > ( TickType_t ) 0 )
{
/* Clear bits in the task's notification value as bits may get
* set by the notifying task or interrupt. This can be used to
* clear the value to zero. */
pxCurrentTCB->ulNotifiedValue[ uxIndexToWait ] &= ~ulBitsToClearOnEntry;
traceTASK_NOTIFY_WAIT_BLOCK( uxIndexToWait );

/* Mark this task as waiting for a notification. */
pxCurrentTCB->ucNotifyState[ uxIndexToWait ] = taskWAITING_NOTIFICATION;
/* Suspend the scheduler before enabling interrupts. */
vTaskSuspendAll();
taskEXIT_CRITICAL();

if( xTicksToWait > ( TickType_t ) 0 )
{
prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );
traceTASK_NOTIFY_WAIT_BLOCK( uxIndexToWait );
prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );

/* All ports are written to allow a yield in a critical
* section (some will yield immediately, others wait until the
* critical section exits) - but it is not something that
* application code should ever do. */
if( xTaskResumeAll() == pdFALSE )
{
portYIELD_WITHIN_API();
}
else
Expand All @@ -4848,10 +4857,13 @@ TickType_t uxTaskResetEventItemValue( void )
}
else
{
mtCOVERAGE_TEST_MARKER();
taskEXIT_CRITICAL();
}
}
taskEXIT_CRITICAL();
else
{
taskEXIT_CRITICAL();
}

taskENTER_CRITICAL();
{
Expand Down