Fix xTaskNotifyWait & ulTaskNotifyTake determinism. (#833)

This PR fixes the bug described in the following issue:
#612.
This was originally contributed in the following PR:
#625.

The implementation suspends the scheduler before exiting the
critical section (i.e. before enabling interrupts). If we do not do
so, a notification sent from an ISR, which happens after exiting
the critical section and before suspending the scheduler, will
get lost. The sequence of events will be:

1. Exit critical section.
2. Interrupt - ISR calls xTaskNotifyFromISR which adds the task to
    the Ready list.
3. Suspend scheduler.
4. prvAddCurrentTaskToDelayedList moves the task to the delayed
    or suspended list.
5. Resume scheduler does not touch the task (because it is not on
    the pendingReady list), effectively losing the notification from
    the ISR.

The same does not happen when we suspend the scheduler before
exiting the critical section. The sequence of events in this case will
be:

1. Suspend scheduler.
2. Exit critical section.
3. Interrupt - ISR calls xTaskNotifyFromISR which adds the task to
    the pendingReady list as the scheduler is suspended.
4. prvAddCurrentTaskToDelayedList adds the task to delayed or
    suspended list. Note that this operation does not nullify the add
    to pendingReady list done in the above step because a different
    list item, namely xEventListItem, is used for adding the task to the
    pendingReady list. In other words, the task still remains on the
    pendingReady list.
5. Resume scheduler moves the task from pendingReady list to the Ready list.

------------

Co-authored-by: Jacob Carver <[email protected]>

Author: kar-rahul-aws

tasks.c

@@ -5198,7 +5198,7 @@ void vTaskPlaceOnEventList( List_t * const pxEventList,
 
     configASSERT( pxEventList );
 
-    /* THIS FUNCTION MUST BE CALLED WITH EITHER INTERRUPTS DISABLED OR THE
+    /* THIS FUNCTION MUST BE CALLED WITH THE
      * SCHEDULER SUSPENDED AND THE QUEUE BEING ACCESSED LOCKED. */
 
     /* Place the event list item of the TCB in the appropriate event list.
@@ -7480,28 +7480,68 @@ TickType_t uxTaskResetEventItemValue( void )
                                       TickType_t xTicksToWait )
     {
         uint32_t ulReturn;
+        BaseType_t xAlreadyYielded;
 
         traceENTER_ulTaskGenericNotifyTake( uxIndexToWaitOn, xClearCountOnExit, xTicksToWait );
 
         configASSERT( uxIndexToWaitOn < configTASK_NOTIFICATION_ARRAY_ENTRIES );
 
         taskENTER_CRITICAL();
+
+        /* Only block if the notification count is not already non-zero. */
+        if( pxCurrentTCB->ulNotifiedValue[ uxIndexToWaitOn ] == 0UL )
         {
-            /* Only block if the notification count is not already non-zero. */
-            if( pxCurrentTCB->ulNotifiedValue[ uxIndexToWaitOn ] == 0UL )
+            /* Mark this task as waiting for a notification. */
+            pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] = taskWAITING_NOTIFICATION;
+
+            if( xTicksToWait > ( TickType_t ) 0 )
             {
-                /* Mark this task as waiting for a notification. */
-                pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] = taskWAITING_NOTIFICATION;
+                traceTASK_NOTIFY_TAKE_BLOCK( uxIndexToWaitOn );
 
-                if( xTicksToWait > ( TickType_t ) 0 )
+                /* We MUST suspend the scheduler before exiting the critical
+                 * section (i.e. before enabling interrupts).
+                 *
+                 * If we do not do so, a notification sent from an ISR, which
+                 * happens after exiting the critical section and before
+                 * suspending the scheduler, will get lost. The sequence of
+                 * events will be:
+                 * 1. Exit critical section.
+                 * 2. Interrupt - ISR calls xTaskNotifyFromISR which adds the
+                 *    task to the Ready list.
+                 * 3. Suspend scheduler.
+                 * 4. prvAddCurrentTaskToDelayedList moves the task to the
+                 *    delayed or suspended list.
+                 * 5. Resume scheduler does not touch the task (because it is
+                 *    not on the pendingReady list), effectively losing the
+                 *    notification from the ISR.
+                 *
+                 * The same does not happen when we suspend the scheduler before
+                 * exiting the critical section. The sequence of events in this
+                 * case will be:
+                 * 1. Suspend scheduler.
+                 * 2. Exit critical section.
+                 * 3. Interrupt - ISR calls xTaskNotifyFromISR which adds the
+                 *    task to the pendingReady list as the scheduler is
+                 *    suspended.
+                 * 4. prvAddCurrentTaskToDelayedList adds the task to delayed or
+                 *    suspended list. Note that this operation does not nullify
+                 *    the add to pendingReady list done in the above step because
+                 *    a different list item, namely xEventListItem, is used for
+                 *    adding the task to the pendingReady list. In other words,
+                 *    the task still remains on the pendingReady list.
+                 * 5. Resume scheduler moves the task from pendingReady list to
+                 *    the Ready list.
+                 */
+                vTaskSuspendAll();
                 {
+                    taskEXIT_CRITICAL();
+
                     prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );
-                    traceTASK_NOTIFY_TAKE_BLOCK( uxIndexToWaitOn );
+                }
+                xAlreadyYielded = xTaskResumeAll();
 
-                    /* 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( xAlreadyYielded == pdFALSE )
+                {
                     taskYIELD_WITHIN_API();
                 }
                 else
@@ -7511,10 +7551,13 @@ TickType_t uxTaskResetEventItemValue( void )
             }
             else
             {
-                mtCOVERAGE_TEST_MARKER();
+                taskEXIT_CRITICAL();
             }
         }
-        taskEXIT_CRITICAL();
+        else
+        {
+            taskEXIT_CRITICAL();
+        }
 
         taskENTER_CRITICAL();
         {
@@ -7557,34 +7600,73 @@ TickType_t uxTaskResetEventItemValue( void )
                                        uint32_t * pulNotificationValue,
                                        TickType_t xTicksToWait )
     {
-        BaseType_t xReturn;
+        BaseType_t xReturn, xAlreadyYielded;
 
         traceENTER_xTaskGenericNotifyWait( uxIndexToWaitOn, ulBitsToClearOnEntry, ulBitsToClearOnExit, pulNotificationValue, xTicksToWait );
 
         configASSERT( uxIndexToWaitOn < configTASK_NOTIFICATION_ARRAY_ENTRIES );
 
         taskENTER_CRITICAL();
+
+        /* Only block if a notification is not already pending. */
+        if( pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] != taskNOTIFICATION_RECEIVED )
         {
-            /* Only block if a notification is not already pending. */
-            if( pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] != 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[ uxIndexToWaitOn ] &= ~ulBitsToClearOnEntry;
+            /* 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[ uxIndexToWaitOn ] &= ~ulBitsToClearOnEntry;
 
-                /* Mark this task as waiting for a notification. */
-                pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] = taskWAITING_NOTIFICATION;
+            /* Mark this task as waiting for a notification. */
+            pxCurrentTCB->ucNotifyState[ uxIndexToWaitOn ] = taskWAITING_NOTIFICATION;
 
-                if( xTicksToWait > ( TickType_t ) 0 )
+            if( xTicksToWait > ( TickType_t ) 0 )
+            {
+                traceTASK_NOTIFY_WAIT_BLOCK( uxIndexToWaitOn );
+
+                /* We MUST suspend the scheduler before exiting the critical
+                 * section (i.e. before enabling interrupts).
+                 *
+                 * If we do not do so, a notification sent from an ISR, which
+                 * happens after exiting the critical section and before
+                 * suspending the scheduler, will get lost. The sequence of
+                 * events will be:
+                 * 1. Exit critical section.
+                 * 2. Interrupt - ISR calls xTaskNotifyFromISR which adds the
+                 *    task to the Ready list.
+                 * 3. Suspend scheduler.
+                 * 4. prvAddCurrentTaskToDelayedList moves the task to the
+                 *    delayed or suspended list.
+                 * 5. Resume scheduler does not touch the task (because it is
+                 *    not on the pendingReady list), effectively losing the
+                 *    notification from the ISR.
+                 *
+                 * The same does not happen when we suspend the scheduler before
+                 * exiting the critical section. The sequence of events in this
+                 * case will be:
+                 * 1. Suspend scheduler.
+                 * 2. Exit critical section.
+                 * 3. Interrupt - ISR calls xTaskNotifyFromISR which adds the
+                 *    task to the pendingReady list as the scheduler is
+                 *    suspended.
+                 * 4. prvAddCurrentTaskToDelayedList adds the task to delayed or
+                 *    suspended list. Note that this operation does not nullify
+                 *    the add to pendingReady list done in the above step because
+                 *    a different list item, namely xEventListItem, is used for
+                 *    adding the task to the pendingReady list. In other words,
+                 *    the task still remains on the pendingReady list.
+                 * 5. Resume scheduler moves the task from pendingReady list to
+                 *    the Ready list.
+                 */
+                vTaskSuspendAll();
                 {
+                    taskEXIT_CRITICAL();
+
                     prvAddCurrentTaskToDelayedList( xTicksToWait, pdTRUE );
-                    traceTASK_NOTIFY_WAIT_BLOCK( uxIndexToWaitOn );
+                }
+                xAlreadyYielded = xTaskResumeAll();
 
-                    /* 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( xAlreadyYielded == pdFALSE )
+                {
                     taskYIELD_WITHIN_API();
                 }
                 else
@@ -7594,10 +7676,13 @@ TickType_t uxTaskResetEventItemValue( void )
             }
             else
             {
-                mtCOVERAGE_TEST_MARKER();
+                taskEXIT_CRITICAL();
             }
         }
-        taskEXIT_CRITICAL();
+        else
+        {
+            taskEXIT_CRITICAL();
+        }
 
         taskENTER_CRITICAL();
         {