dereferencing pointer to incomplete type ‘struct workqueue_struct’

[John Busch <jbusch175 at gmail dot com>]

Hello,

I’m new to SystemTap, and trying to print information about kernel
work queues.  I’ve used the example code in section 4.3 [1] of the
tutorial as my starting point, and I’m working on an up-to-date Fedora
25 system.

The example code in the tutorial works just fine.  However, when I
essentially swap out variable and struct names to modify the example
to walk through workqueue_structs (instead of task_structs), I get a
“dereferencing pointer to incomplete type ‘struct workqueue_struct’”
compilation error on the line containing list_for_each_safe.  Code
sample below.

workqueue_struct is defined in workqueue.h [2] and workqueue.c [4],
and I’m trying to access the global variable system_wq [3, 5].  How
would I gain access to these objects in systemtap?  I must be doing
something wrong, but can't figure out what it is.

Thanks!

[1] https://sourceware.org/systemtap/tutorial/Tapsets.html
[2] http://lxr.free-electrons.com/source/include/linux/workqueue.h#L16
[3] http://lxr.free-electrons.com/source/include/linux/workqueue.h#L365
[4] http://lxr.free-electrons.com/source/kernel/workqueue.c#L239
[5] http://lxr.free-electrons.com/source/kernel/workqueue.c#L338

%{
#include <linux/workqueue.h>
#include <linux/list.h>
%}

function read_wq_list:long ()
%{
   int ret = 0;
   struct workqueue_struct *wq;
   struct list_head *p, *n;

   list_for_each_safe(p, n, system_wq->list) {
       wq = list_entry(p, struct workqueue_struct, list);
       _stp_printf("workqueue struct %s\n", wq->name);
   }

   STAP_RETURN(ret);
%}

probe begin
{
printf("reading list of workqueues\n")
read_wq_list()
exit()
}