diff --git a/src/xtimer.rs b/src/xtimer.rs
index 8249fbd..895bfd6 100644
--- a/src/xtimer.rs
+++ b/src/xtimer.rs
@@ -30,6 +30,7 @@ pub struct XTimerAttacher {
     timers: TimerListListHeadRp,
 }
 
+#[derive(Clone)]
 enum TimerType {
     Timeout,
     Interval
@@ -255,8 +256,7 @@ impl XTimer {
                 }
             self.timer_tick += 1;
 
-            //println!("timer_tick:{} index:{} empty:{}", self.timer_tick, index, self.tv1[index].borrow().empty());
-            crate::ListHead::<TimerList>::replace_init
+             crate::ListHead::<TimerList>::replace_init
                 (&self.tv1[index],
                  &self.work_list);
             return true;
@@ -270,25 +270,34 @@ impl XTimer {
         let work_list = this.borrow().work_list.clone();
         while this.borrow_mut().fetch_work_list(tick) {
             while !work_list.borrow().empty() {
-                let timer = &work_list.borrow().first_entry();
-                this.borrow_mut().running_timer = timer.clone();
-                match &mut timer.upgrade().unwrap().borrow_mut().cb {
-                    Some(v) => {
-                        (*v)(TimerEvent::Exec, None);
-                    }
-                    None => {
+                {
+                    let timer = &work_list.borrow().first_entry();
+                    this.borrow_mut().running_timer = timer.clone();
+                    match &mut timer.upgrade().unwrap().borrow_mut().cb {
+                        Some(v) => {
+                            (*v)(TimerEvent::Exec, None);
+                        }
+                        None => {
+                        }
                     }
                 }
                 let running_timer = this.borrow().running_timer.clone();
                 match running_timer.upgrade() {
                     Some(v) => {
-                        match v.borrow().timer_type {
+                        let timer_wp = Rc::downgrade(&v.borrow().wp.clone().unwrap());
+                        let timer_type = v.borrow().timer_type.clone();
+                        let expire_time = v.borrow().expire_time;
+                        match timer_type {
                             TimerType::Timeout => {
-                                this.borrow_mut().internal_delete(Rc::downgrade(&v.borrow().wp.clone().unwrap()), false);
+                                this.borrow_mut().internal_delete
+                                    (Rc::downgrade(&v.borrow().wp.clone().unwrap()),
+                                     false);
                             }
                             TimerType::Interval => {
-                                this.borrow_mut().modify(Rc::downgrade(&v.borrow().wp.clone().unwrap()),
-                                            v.borrow().expire_time);
+                                this.borrow_mut().internal_modify(
+                                    timer_wp,
+                                    expire_time
+                                );
                             }
                         }
                     }
@@ -441,13 +450,14 @@ impl XTimer {
         }
     }
 
-    fn internal_modify(&mut self, timer_wp: XTimerWp, expire_time: i32) {
+    fn internal_modify(&self, timer_wp: XTimerWp, expire_time: i32) {
         self.detach_timer(&timer_wp.upgrade().unwrap());
-        timer_wp.upgrade().unwrap().borrow_mut().expire_time = expire_time;
-        timer_wp.upgrade().unwrap().borrow_mut().expires =
+        let timer = timer_wp.clone().upgrade().unwrap().clone();
+        timer.borrow_mut().expire_time = expire_time;
+        timer.borrow_mut().expires =
             (*self.get_tick_count.as_ref().unwrap())() +
             expire_time as i64;
-        self.internal_add2(&timer_wp.upgrade().unwrap());
+        self.internal_add2(&timer);
     }
 
     pub fn set_timeout(&mut self, time: i32, cb: TimerCb) -> XTimerWp {
@@ -490,11 +500,11 @@ impl XTimer {
     pub fn modify(&mut self, timer_wp: XTimerWp, expire_time: i32) {
         match timer_wp.upgrade() {
             Some(_) => {
-                self.internal_modify(timer_wp, expire_time);
             }
             None => {
             }
         }
+        self.internal_modify(timer_wp, expire_time);
     }
 
     pub fn delete_current_timer(&mut self) {