score: PR2181: Add _Thread_Yield()

The _Scheduler_Yield() was called by the executing thread with thread
dispatching disabled and interrupts enabled.  The rtems_task_suspend()
is explicitly allowed in ISRs:

http://rtems.org/onlinedocs/doc-current/share/rtems/html/c_user/Interrupt-Manager-Directives-Allowed-from-an-ISR.html#Interrupt-Manager-Directives-Allowed-from-an-ISR

Unlike the other scheduler operations the locking was performed inside
the operation.  This lead to the following race condition.  Suppose a
ISR suspends the executing thread right before the yield scheduler
operation.  Now the executing thread is not longer in the set of ready
threads.  The typical scheduler operations did not check the thread
state and will now extract the thread again and enqueue it.  This
corrupted data structures.

Add _Thread_Yield() and do the scheduler yield operation with interrupts
disabled.  This has a negligible effect on the interrupt latency.

1 files changed, 41 insertions, 0 deletions

diff --git a/cpukit/score/src/threadyield.c b/cpukit/score/src/threadyield.c
new file mode 100644
index 0000000000..b49e2b30ba
--- /dev/null
+++ b/cpukit/score/src/threadyield.c
@@ -0,0 +1,41 @@
+/**
+ * @file
+ *
+ * @brief Thread Yield
+ *
+ * @ingroup ScoreThread
+ */
+
+/*
+ * Copyright (c) 2014 embedded brains GmbH.  All rights reserved.
+ *
+ *  embedded brains GmbH
+ *  Dornierstr. 4
+ *  82178 Puchheim
+ *  Germany
+ *  <rtems@embedded-brains.de>
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rtems.org/license/LICENSE.
+ */
+
+#if HAVE_CONFIG_H
+  #include "config.h"
+#endif
+
+#include <rtems/score/threadimpl.h>
+#include <rtems/score/schedulerimpl.h>
+
+void _Thread_Yield( Thread_Control *executing )
+{
+  ISR_Level level;
+
+  _ISR_Disable( level );
+
+  if ( _States_Is_ready( executing->current_state ) ) {
+    _Scheduler_Yield( _Scheduler_Get( executing ), executing );
+  }
+
+  _ISR_Enable( level );
+}