linux-kernel - Re: [PATCH v3 07/22] kthread: Detect when a kthread work is used by more workers

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Message-ID: <20151124144942.GC17033@mtj.duckdns.org>
Date:	Tue, 24 Nov 2015 09:49:42 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Vlastimil Babka <vbabka@...e.cz>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 07/22] kthread: Detect when a kthread work is used by
 more workers

Hello, Petr.

On Tue, Nov 24, 2015 at 11:06:50AM +0100, Petr Mladek wrote:
> > > @@ -610,6 +625,12 @@ repeat:
> > >  	if (work) {
> > >  		__set_current_state(TASK_RUNNING);
> > >  		work->func(work);
> > > +
> > > +		spin_lock_irq(&worker->lock);
> > > +		/* Allow to queue the work into another worker */
> > > +		if (!kthread_work_pending(work))
> > > +			work->worker = NULL;
> > > +		spin_unlock_irq(&worker->lock);
> > 
> > Doesn't this mean that the work item can't be freed from its callback?
> > That pattern tends to happen regularly.
> 
> I am not sure if I understand your question. Do you mean switching
> work->func during the life time of the struct kthread_work? This
> should not be affected by the above code.

So, something like the following.

void my_work_fn(work)
{
	struct my_struct *s = container_of(work, ...);

	do something with s;
	kfree(s);
}

and the queuer does

	struct my_struct *s = kmalloc(sizeof(*s));

	init s and s->work;
	queue(&s->work);

expecting s to be freed on completion.  IOW, you can't expect the work
item to remain accessible once the work function starts executing.

> The above code allows to queue an _unused_ kthread_work into any
> kthread_worker. For example, it is needed for khugepaged,
> see http://marc.info/?l=linux-kernel&m=144785344924871&w=2
> The work is static but the worker can be started/stopped
> (allocated/freed) repeatedly. It means that the work need
> to be usable with many workers. But it is associated only
> with one worker when being used.

It can just re-init work items when it restarts workers, right?

Thanks.

-- 
tejun
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