tmr: prevent timer race condition on cancel

If `tmr_cancel` is called from two threads it's enough to cancel once.

include/re_tmr.h

@@ -6,6 +6,7 @@
 
 
 #include "re_thread.h"
+#include "re_atomic.h"
 
 /**
  * Defines the timeout handler
@@ -19,6 +20,7 @@ struct tmrl;
 /** Defines a timer */
 struct tmr {
 	struct le le;       /**< Linked list element */
+	RE_ATOMIC bool active; /**< Timer is active  */
 	mtx_t *lock;        /**< Mutex lock          */
 	tmr_h *th;          /**< Timeout handler     */
 	void *arg;          /**< Handler argument    */

src/tmr/tmr.c

@@ -393,6 +393,11 @@ static void tmr_startcont_dbg(struct tmr *tmr, uint64_t delay, bool syncnow,
 	if (!tmr || !tmrl)
 		return;
 
+	if (!re_atomic_acq(&tmr->active) && !th)
+		return;
+
+	re_atomic_rls_set(&tmr->active, false);
+
 	if (!tmr->lock || !tmr->le.list)
 		lock = tmrl->lock;
 	else
@@ -445,6 +450,8 @@ static void tmr_startcont_dbg(struct tmr *tmr, uint64_t delay, bool syncnow,
 		}
 	}
 
+	re_atomic_rls_set(&tmr->active, true);
+
 	mtx_unlock(lock);
 }