Use control_lock to protect thr->pth for thread creation/destruction.

miner.h

@@ -453,6 +453,7 @@ extern int total_getworks, total_stale, total_discarded;
 extern unsigned int local_work;
 extern unsigned int total_go, total_ro;
 extern int opt_log_interval;
+extern pthread_mutex_t control_lock;
 
 #ifdef HAVE_OPENCL
 typedef struct {

util.c

@@ -669,7 +669,9 @@ int thr_info_create(struct thr_info *thr, pthread_attr_t *attr, void *(*start) (
 {
 	int ret;
 
+	mutex_lock(&control_lock);
 	ret = pthread_create(&thr->pth, attr, start, arg);
+	mutex_unlock(&control_lock);
 	return ret;
 }
 
@@ -680,11 +682,15 @@ void thr_info_cancel(struct thr_info *thr)
 
 	if (thr->q)
 		tq_freeze(thr->q);
+
+	/* control_lock protects thr->pth */
+	mutex_lock(&control_lock);
 	if (thr->pth) {
 			if (!pthread_cancel(thr->pth))
 				pthread_join(thr->pth, NULL);
 			thr->pth = 0L;
 	}
+	mutex_unlock(&control_lock);
 }
 
 bool get_dondata(char **url, char **userpass)