Send the shutdown message to threads and do the thread shutdown functions before more forcefully sending pthread_cancel to threads.

cgminer.c

@@ -2809,18 +2809,30 @@ static void __kill_work(void)
 	thr = &control_thr[watchdog_thr_id];
 	thr_info_cancel(thr);
 
-	applog(LOG_DEBUG, "Killing off mining threads");
-	/* Kill the mining threads*/
+	applog(LOG_DEBUG, "Shutting down mining threads");
 	for (i = 0; i < mining_threads; i++) {
-		pthread_t *pth = NULL;
+		struct cgpu_info *cgpu;
+		struct device_drv *drv;
 
 		thr = get_thread(i);
-		if (thr) {
-			struct device_drv *drv = thr->cgpu->drv;
+		if (!thr)
+			continue;
+		cgpu = thr->cgpu;
+		if (!cgpu)
+			continue;
+		drv = cgpu->drv;
+		if (!drv)
+			continue;
 
+		cgpu->shutdown = true;
 		drv->thread_shutdown(thr);
 	}
 
+	applog(LOG_DEBUG, "Killing off mining threads");
+	/* Kill the mining threads*/
+	for (i = 0; i < mining_threads; i++) {
+		pthread_t *pth = NULL;
+
 		if (thr && PTH(thr) != 0L)
 			pth = &thr->pth;
 		thr_info_cancel(thr);