mt: Fix preemption

Preemption was supposed to be supported, but it had no means of safely
updating the state of a thread, so mt_exec() could fail to resume a
preempted thread.

mt_exec() is allowed to be called only from the main Contiki thread, so
the mt threads passed to it may be only ready or exited, not running.
Consequently, there is no need for a distinction between the ready and
running states, so merge them as a started state, which avoids having to
update the state of a thread upon preemption.

Signed-off-by: Benoît Thébaudeau <[email protected]>

mt.c

@@ -68,14 +68,13 @@ mt_start(struct mt_thread *thread, void (* function)(void *), void *data)
      stack with the correct parameters. */
   mtarch_start(&thread->thread, function, data);
 
-  thread->state = MT_STATE_READY;
+  thread->state = MT_STATE_STARTED;
 }
 /*--------------------------------------------------------------------------*/
 void
 mt_exec(struct mt_thread *thread)
 {
-  if(thread->state == MT_STATE_READY) {
-    thread->state = MT_STATE_RUNNING;
+  if(thread->state == MT_STATE_STARTED) {
     current = thread;
     /* Switch context to the thread. The function call will not return
        until the the thread has yielded, or is preempted. */
@@ -87,22 +86,18 @@ void
 mt_yield(void)
 {
   mtarch_pstop();
-  current->state = MT_STATE_READY;
-  current = NULL;
   /* This function is called from the running thread, and we call the
      switch function in order to switch the thread to the main Contiki
      program instead. For us, the switch function will not return
      until the next time we are scheduled to run. */
   mtarch_yield();
-
 }
 /*--------------------------------------------------------------------------*/
 void
 mt_exit(void)
 {
   mtarch_pstop();
   current->state = MT_STATE_EXITED;
-  current = NULL;
   mtarch_yield();
 }
 /*--------------------------------------------------------------------------*/

mt.h

@@ -84,9 +84,8 @@
 
 #include "contiki.h"
 
-#define MT_STATE_READY   1
-#define MT_STATE_RUNNING 2
-#define MT_STATE_EXITED  5
+#define MT_STATE_STARTED 1
+#define MT_STATE_EXITED  2
 
 /**
  * An opaque structure that is used for holding the state of a thread.