Re: dereferencing pointer to incomplete type ‘struct workqueue_struct’

[David Smith <dsmith at redhat dot com>]

If you look carefully, workqueue_struct isn't defined in
include/linux/workqueue.h. It is declared ("struct
workqueue_struct;"), but never fully defined ("struct workqueue_struct
{ char *name, ... }") in that file. So, that's why the compiler is
giving you that error. It has nothing to do with systemtap, if you
tried to compile a kernel module without systemtap, you'd get the same
error. The workqueue_struct definition is private to the workqueue.c
file.

Strangely enough, systemtap use of debuginfo would give it a
definition of that structure. So, you could write something like the
following in script language:

====
function display_wq_name:long(wq:long)
{
    printf("workqueue struct %s\n", @cast(wq, "workqueue_struct")->name)
}
====

However, the problem is iterating through that list in script
language. To do that correctly, you'd need embedded-C as you've got
here.

To solve your problem, unfortunately copying the workqueue_struct
definition into your module is the only way to go to make the compiler
happy. Of course if you plan to use this long term, you'll have to
keep an eye on that private structure and be on the lookout for
changes.

Also note that if this is just a one-off exercise, your code is fine.
If you planned to use this long term or on a system you really cared
about, you'd need to change it to use systemtap's kderef_* functions
to ensure that memory is safe to read.


On Thu, Apr 27, 2017 at 8:02 AM, John Busch <jbusch175@gmail.com> wrote:
> This worked for me.  Thank you Arkady!
>
> On Thu, Apr 27, 2017 at 3:07 AM, Arkady <arkady.miasnikov@gmail.com> wrote:
>> The simplest way to resolve the problem is top copy the struct to the
>> inline C code
>>
>> http://lxr.free-electrons.com/source/kernel/workqueue.c#L239
>>
>> On Wed, Apr 26, 2017 at 6:20 PM, John Busch <jbusch175@gmail.com> wrote:
>>> 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()
>>> }



-- 
David Smith
Principal Software Engineer
Red Hat