sys: xtimer: fix some race conditions

sys/include/xtimer.h

@@ -251,11 +251,8 @@ void xtimer_set(xtimer_t *timer, uint32_t offset);
  * @note this function runs in O(n) with n being the number of active timers
  *
  * @param[in] timer ptr to timer structure that will be removed
- *
- * @return  1 on success
- * @return  0 when timer was not active
  */
-int xtimer_remove(xtimer_t *timer);
+void xtimer_remove(xtimer_t *timer);
 
 /**
  * @brief receive a message blocking but with timeout

sys/xtimer/xtimer_core.c

@@ -40,6 +40,7 @@ static xtimer_t *long_list_head = NULL;
 static void _add_timer_to_list(xtimer_t **list_head, xtimer_t *timer);
 static void _add_timer_to_long_list(xtimer_t **list_head, xtimer_t *timer);
 static void _shoot(xtimer_t *timer);
+static void _remove(xtimer_t *timer);
 static inline void _lltimer_set(uint32_t target);
 static uint32_t _time_left(uint32_t target, uint32_t reference);
 
@@ -94,7 +95,10 @@ void _xtimer_set64(xtimer_t *timer, uint32_t offset, uint32_t long_offset)
         xtimer_set(timer, (uint32_t) offset);
     }
     else {
-        xtimer_remove(timer);
+        int state = disableIRQ();
+        if (_is_set(timer)) {
+            _remove(timer);
+        }
 
         _xtimer_now64(&timer->target, &timer->long_target);
         timer->target += offset;
@@ -103,7 +107,6 @@ void _xtimer_set64(xtimer_t *timer, uint32_t offset, uint32_t long_offset)
             timer->long_target++;
         }
 
-        int state = disableIRQ();
         _add_timer_to_long_list(&long_list_head, timer);
         restoreIRQ(state);
         DEBUG("xtimer_set64(): added longterm timer (long_target=%" PRIu32 " target=%" PRIu32 ")\n",
@@ -173,13 +176,17 @@ int _xtimer_set_absolute(xtimer_t *timer, uint32_t target)
         return 0;
     }
 
+    unsigned state = disableIRQ();
+    if (_is_set(timer)) {
+        _remove(timer);
+    }
+
     timer->target = target;
     timer->long_target = _long_cnt;
     if (target < now) {
         timer->long_target++;
     }
 
-    unsigned state = disableIRQ();
     if ( (timer->long_target > _long_cnt) || !_this_high_period(target) ) {
         DEBUG("xtimer_set_absolute(): the timer doesn't fit into the low-level timer's mask.\n");
         _add_timer_to_long_list(&long_list_head, timer);
@@ -240,14 +247,8 @@ static int _remove_timer_from_list(xtimer_t **list_head, xtimer_t *timer)
     return 0;
 }
 
-int xtimer_remove(xtimer_t *timer)
+static void _remove(xtimer_t *timer)
 {
-    if (!_is_set(timer)) {
-        return 0;
-    }
-
-    unsigned state = disableIRQ();
-    int res = 0;
     if (timer_list_head == timer) {
         uint32_t next;
         timer_list_head = timer->next;
@@ -261,17 +262,21 @@ int xtimer_remove(xtimer_t *timer)
         _lltimer_set(next);
     }
     else {
-        res = _remove_timer_from_list(&timer_list_head, timer) ||
-            _remove_timer_from_list(&overflow_list_head, timer) ||
-            _remove_timer_from_list(&long_list_head, timer);
+        if (!_remove_timer_from_list(&timer_list_head, timer)) {
+            if (!_remove_timer_from_list(&overflow_list_head, timer)) {
+                _remove_timer_from_list(&long_list_head, timer);
+            }
+        }
     }
+}
 
-    timer->target = 0;
-    timer->long_target = 0;
-
+void xtimer_remove(xtimer_t *timer)
+{
+    int state = disableIRQ();
+    if (_is_set(timer)) {
+        _remove(timer);
+    }
     restoreIRQ(state);
-
-    return res;
 }
 
 static uint32_t _time_left(uint32_t target, uint32_t reference)