commit a32189c495cf4dbb71fa497adcaa2ab31aad7021
Author: Martin Cermak <mcermak@redhat.com>
Date:   Wed Jun 29 19:03:11 2016 +0200

    Avoid null pointer exception in the ioscheduler.elv_add_request probe.
    
    This update makes the ioscheduler.elv_add_request probe gracefully handle
    situation where $q->elevator is NULL (RHBZ1269062).

diff --git a/tapset/linux/ioscheduler.stp b/tapset/linux/ioscheduler.stp
index 00d75a5..121fde8 100644
--- a/tapset/linux/ioscheduler.stp
+++ b/tapset/linux/ioscheduler.stp
@@ -149,7 +149,7 @@ probe ioscheduler.elv_add_request.tp = kernel.trace("block_rq_insert") ?
 {
 	name = "elv_add_request"
 	q =  $q
-	elevator_name = kernel_string(
+	elevator_name = ($q->elevator == 0) ? "" : kernel_string(
 		@choose_defined($q->elevator->type->elevator_name,
 			@choose_defined($q->elevator->elevator_type->elevator_name,
                                         $q->elevator->elevator_name)), "")