linux-kernel - Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper

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Date:	Thu, 22 Mar 2012 15:27:58 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	"Rafael J. Wysocki" <rjw@...k.pl>, keyrings@...ux-nfs.org,
	linux-security-module@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	NFS list <linux-nfs@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	"Bhamare, Sachin" <sbhamare@...asas.com>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Kay Sievers <kay.sievers@...y.org>,
	James Morris <jmorris@...ei.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>, David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper_timeout()
	API

On 03/21, Boaz Harrosh wrote:
>
> > @@ -258,7 +262,8 @@ static void __call_usermodehelper(struct work_struct *work)
> >
> >  	switch (wait) {
> >  	case UMH_NO_WAIT:
> > -		call_usermodehelper_freeinfo(sub_info);
> > +		kref_put(&sub_info->kref,  call_usermodehelper_freeinfo);
> > +		kref_put(&sub_info->kref,  call_usermodehelper_freeinfo);
> >  		break;

This doesn't look very nice. If you add the refcounting, it should be
consistent. Imho it is better to change call_usermodehelper_exec() so
that UMH_NO_WAIT does kref_put() too. Just s/goto unlock/goto out/ afaics.

> > @@ -452,22 +459,27 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
> >
> >  	sub_info->complete = &done;
> >  	sub_info->wait = wait;
> > +	if (!sub_info->wait_timeout)
> > +		sub_info->wait_timeout = MAX_SCHEDULE_TIMEOUT;
> >
> > +	/* Balanced in __call_usermodehelper or wait_for_helper */
> > +	kref_get(&sub_info->kref);
> >  	queue_work(khelper_wq, &sub_info->work);
> >  	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
> >  		goto unlock;
> > -	wait_for_completion(&done);
> > -	retval = sub_info->retval;
> > -
> > +	if (likely(wait_for_completion_timeout(&done, sub_info->wait_timeout)))
> > +		retval = sub_info->retval;
> > +	else
> > +		retval = -ETIMEDOUT;
> >  out:
> > -	call_usermodehelper_freeinfo(sub_info);
> > +	kref_put(&sub_info->kref, call_usermodehelper_freeinfo);
> >  unlock:
> >  	helper_unlock();
> >  	return retval;
> >  }

This looks obviously wrong. You also need to move *sub_info->complete
into subprocess_info.

> Author: Oleg Nesterov <oleg@...hat.com>
> Date:   Wed Mar 21 10:57:41 2012 +1100
>
>     usermodehelper: implement UMH_KILLABLE
>
>     Implement UMH_KILLABLE, should be used along with UMH_WAIT_EXEC/PROC.  The
>     caller must ensure that subprocess_info->path/etc can not go away until
>     call_usermodehelper_freeinfo().
> ...
>
> I think that my patch above does a much better/cleaner lifetime management of the
> subprocess_info struct, with the use of a kref.

This is subjective, you know ;) I specially tried to avoid the
refcounting.

In any case. I do not know why do we need timeout, but this is
orthogonal to KILLABLE. Please redo your patches on top of -mm
tree? Please note that in this case the change becomes trivial.

And please explain the use-case for the new API.

> Anyway I thought that we are not
> suppose to use xhcg() since it is not portable to all ARCHs. ;-)

Hmm. For example, exit_mm() does xchg().

Oleg.

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