Add support for newlib dynamic reentrancy (#496)

Previously, newlib's _impure_ptr was updated on every context switch
to point to the current task's _reent structure.

However, this behavior is no longer valid on multi-core systems due
to the fact that multiple cores can switch contexts at the same time,
thus leading to the corruption of the _impure_ptr.

However, Newlib can be compiled with __DYNAMIC_REENT__ enabled which
will cause newlib functions to call __getreent() instead in order to
obtain the required reent struct.

This commit adds dynamic reentrancy support to FreeRTOS:

- Added a configNEWLIB_REENTRANT_IS_DYNAMIC to enable dynamic reentrancy support
- _impure_ptr is no longer updated with reentrancy is dynamic
- Port must provide their own __getreent() that returns the current task's reent struct

2 files changed, 20 insertions, 6 deletions

diff --git a/include/FreeRTOS.h b/include/FreeRTOS.h
index 6aa3b803c..60abd75b6 100644
--- a/include/FreeRTOS.h
+++ b/include/FreeRTOS.h
@@ -72,6 +72,14 @@
     #include <reent.h>
 #endif
 
+#ifdef configNEWLIB_REENTRANT_IS_DYNAMIC
+    #if ( configUSE_NEWLIB_REENTRANT != 1 )
+        #error configUSE_NEWLIB_REENTRANT must be defined to 1 to enable configNEWLIB_REENTRANT_IS_DYNAMIC
+    #endif
+#else /* configNEWLIB_REENTRANT_IS_DYNAMIC */
+    #define configNEWLIB_REENTRANT_IS_DYNAMIC   0
+#endif /* configNEWLIB_REENTRANT_IS_DYNAMIC */
+
 /*
  * Check all the required application specific macros have been defined.
  * These macros are application specific and (as downloaded) are defined
diff --git a/tasks.c b/tasks.c
index 4296e300b..7eb1409aa 100644
--- a/tasks.c
+++ b/tasks.c
@@ -2854,15 +2854,18 @@ void vTaskStartScheduler( void )
          * starts to run. */
         portDISABLE_INTERRUPTS();
 
-        #if ( configUSE_NEWLIB_REENTRANT == 1 )
+        #if ( ( configUSE_NEWLIB_REENTRANT == 1 ) && ( configNEWLIB_REENTRANT_IS_DYNAMIC == 0 ) )
             {
                 /* Switch Newlib's _impure_ptr variable to point to the _reent
                  * structure specific to the task that will run first.
                  * See the third party link http://www.nadler.com/embedded/newlibAndFreeRTOS.html
-                 * for additional information. */
+                 * for additional information.
+                 *
+                 * Note: Updating the _impure_ptr is not required when Newlib is compiled with
+                 * __DYNAMIC_REENT__ enabled. The port should provide __getreent() instead. */
                 _impure_ptr = &( pxCurrentTCB->xNewLib_reent );
             }
-        #endif /* configUSE_NEWLIB_REENTRANT */
+        #endif /* ( configUSE_NEWLIB_REENTRANT == 1 ) && ( configNEWLIB_REENTRANT_IS_DYNAMIC == 0 ) */
 
         xNextTaskUnblockTime = portMAX_DELAY;
         xSchedulerRunning = pdTRUE;
@@ -3945,15 +3948,18 @@ void vTaskSwitchContext( BaseType_t xCoreID )
                 }
             #endif
 
-            #if ( configUSE_NEWLIB_REENTRANT == 1 )
+            #if ( ( configUSE_NEWLIB_REENTRANT == 1 ) && ( configNEWLIB_REENTRANT_IS_DYNAMIC == 0 ) )
                 {
                     /* Switch Newlib's _impure_ptr variable to point to the _reent
                      * structure specific to this task.
                      * See the third party link http://www.nadler.com/embedded/newlibAndFreeRTOS.html
-                     * for additional information. */
+                     * for additional information.
+                     *
+                     * Note: Updating the _impure_ptr is not required when Newlib is compiled with
+                     * __DYNAMIC_REENT__ enabled. The the port should provide __getreent() instead. */
                     _impure_ptr = &( pxCurrentTCB->xNewLib_reent );
                 }
-            #endif /* configUSE_NEWLIB_REENTRANT */
+            #endif /* ( configUSE_NEWLIB_REENTRANT == 1 ) && ( configNEWLIB_REENTRANT_IS_DYNAMIC == 0 ) */
         }
     }
     portRELEASE_ISR_LOCK();