MFC r276532 and r276626:

The "cnputs_mtx" mutex must be allowed to recurse. Debug prints and/or
witness printouts in the console driver clients can cause this mutex
to recurse by calls to "printf()" from witness for example. In
particular this can happen if "debug.witness.skipspin=0" is set in the
boot environment.

sys/kern/kern_cons.c

@@ -505,6 +505,13 @@ cnputs(char *p)
 	int unlock_reqd = 0;
 
 	if (use_cnputs_mtx) {
+	  	/*
+		 * NOTE: Debug prints and/or witness printouts in
+		 * console driver clients can cause the "cnputs_mtx"
+		 * mutex to recurse. Simply return if that happens.
+		 */
+		if (mtx_owned(&cnputs_mtx))
+			return;
 		mtx_lock_spin(&cnputs_mtx);
 		unlock_reqd = 1;
 	}