[PATCH] call_usermodehelper needs to wait longer

[PATCH] call_usermodehelper needs to wait longer

[Srivatsa Vaddagiri]

During my testing of call_usermodehelper, I noticed that 
if it is called from a workqueue function, it does really wait
(when asked to) for the usermodehelper to exit.

Patch below should fix it. It has been tested against 2.6.4-rc2-mm1
on a 4-way x86 SMP box.

--- kmod.c.org  2004-03-09 18:52:19.000000000 +0530
+++ kmod.c      2004-03-09 18:52:38.000000000 +0530
@@ -258,10 +258,13 @@
        if (current_is_keventd()) {
                /* We can't wait on keventd! */
                __call_usermodehelper(&sub_info);
-       } else {
+               if (!wait)
+                       goto out;
+       } else
                schedule_work(&work);
-               wait_for_completion(&done);
-       }
+
+       wait_for_completion(&done);
+
 out:
        return sub_info.retval;
 }



-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017

Re: [PATCH] call_usermodehelper needs to wait longer

[Andrew Morton]

Srivatsa Vaddagiri <vatsa@in.ibm.com> wrote:
>
> During my testing of call_usermodehelper, I noticed that 
> if it is called from a workqueue function, it does really wait
> (when asked to) for the usermodehelper to exit.
> 
> Patch below should fix it. It has been tested against 2.6.4-rc2-mm1
> on a 4-way x86 SMP box.
> 
> --- kmod.c.org  2004-03-09 18:52:19.000000000 +0530
> +++ kmod.c      2004-03-09 18:52:38.000000000 +0530
> @@ -258,10 +258,13 @@
>         if (current_is_keventd()) {
>                 /* We can't wait on keventd! */
>                 __call_usermodehelper(&sub_info);
> -       } else {
> +               if (!wait)
> +                       goto out;
> +       } else
>                 schedule_work(&work);
> -               wait_for_completion(&done);
> -       }
> +
> +       wait_for_completion(&done);
> +
>  out:
>         return sub_info.retval;
>  }

I'm not so sure about this.  There are deadlock potentials if the usermode
application wants to perform some function which requires keventd services
to complete - the application cannot complete because keventd is itself
waiting for the application.

Can we think of any circumstances under which keventd _should_
synchronously wait for the userspace app?

btw: your patch had whitespace where the tabs should be (email client
problem), and `patch -p1' format is (much) preferred.

Re: [PATCH] call_usermodehelper needs to wait longer

[Srivatsa Vaddagiri]

On Tue, Mar 09, 2004 at 01:38:35PM -0800, Andrew Morton wrote:
> I'm not so sure about this.  There are deadlock potentials if the usermode
> application wants to perform some function which requires keventd services
> to complete - the application cannot complete because keventd is itself
> waiting for the application.
> 
> Can we think of any circumstances under which keventd _should_
> synchronously wait for the userspace app?

Honestly I don't know ..Would it be reasonable for somebody
to call request_module from a work function (in which case the above
bug is exposed)? I agree it is not a nice thing if we block keventd
waiting for the app to exit. 

> 
> btw: your patch had whitespace where the tabs should be (email client
> problem), and `patch -p1' format is (much) preferred.

Sorry abt that! The whitespace was because of cut-n-paste ..I have
also downloaded your patch scripts and will start using that for 
generating patches henceforth!

-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017

Re: [PATCH] call_usermodehelper needs to wait longer

[Rusty Russell]

On Wed, 2004-03-10 at 19:24, Srivatsa Vaddagiri wrote:
> On Tue, Mar 09, 2004 at 01:38:35PM -0800, Andrew Morton wrote:
> > I'm not so sure about this.  There are deadlock potentials if the usermode
> > application wants to perform some function which requires keventd services
> > to complete - the application cannot complete because keventd is itself
> > waiting for the application.
> > 
> > Can we think of any circumstances under which keventd _should_
> > synchronously wait for the userspace app?
> 
> Honestly I don't know ..Would it be reasonable for somebody
> to call request_module from a work function (in which case the above
> bug is exposed)? I agree it is not a nice thing if we block keventd
> waiting for the app to exit. 

I think a BUG_ON(wait) is arguably right here: anyone trying it will hit
it immediately.  Perhaps a "/* keventd shouldn't block */" comment above
it would help.

Cheers,
-- 
Anyone who quotes me in their signature is an idiot -- Rusty Russell