fix(freertos): Fixed SMP race condition in xStreamBufferSend()

This commit fixes a race condition in dual-core SMP mode where in the
xStreamBufferSend() makes the xTaskWaitingToSend NULL but it may
have already been evaluated to not be NULL by xStreamBufferReceive()
running on another core and eventually leading to a crash in tasks.c.

components/freertos/FreeRTOS-Kernel/idf_changes.md

@@ -125,6 +125,8 @@ The following functions were modified to accommodate SMP behavior:
   - Added extra check to see if current blocking task has already been deleted by the other core.
 - `xStreamBufferReceive()`
   - Added a critical section for setting `xTaskWaitingToReceive` to `NULL` so that the write is SMP safe.
+- `xStreamBufferSend()`
+  - Added a critical section for setting `xTaskWaitingToSend` to `NULL` so that the write is SMP safe.
 
 ### Critical Section Changes
 

components/freertos/FreeRTOS-Kernel/stream_buffer.c

@@ -774,7 +774,18 @@ size_t xStreamBufferSend( StreamBufferHandle_t xStreamBuffer,
 
             traceBLOCKING_ON_STREAM_BUFFER_SEND( xStreamBuffer );
             ( void ) xTaskNotifyWait( ( uint32_t ) 0, ( uint32_t ) 0, NULL, xTicksToWait );
-            pxStreamBuffer->xTaskWaitingToSend = NULL;
+            /* In SMP mode, the task may have been woken and scheduled on
+             * another core. Hence, we must clear the xTaskWaitingToSend
+             * handle in a critical section. */
+            #if ( configNUMBER_OF_CORES > 1 )
+                taskENTER_CRITICAL( &( pxStreamBuffer->xStreamBufferLock ) );
+            #endif /* configNUMBER_OF_CORES > 1 */
+            {
+                pxStreamBuffer->xTaskWaitingToSend = NULL;
+            }
+            #if ( configNUMBER_OF_CORES > 1 )
+                taskEXIT_CRITICAL( &( pxStreamBuffer->xStreamBufferLock ) );
+            #endif /* configNUMBER_OF_CORES > 1 */
         } while( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE );
     }
     else