linux-kernel - Re: [PATCH 2/2] kthread_worker: reimplement flush_kthread_work() to allow freeing the work item being executed

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Message-ID: <1342990015.2487.19.camel@palomino.walls.org>
Date:	Sun, 22 Jul 2012 16:46:54 -0400
From:	Andy Walls <awalls@...metrocast.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	ivtv-devel@...vdriver.org, linux-media@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	spi-devel-general@...ts.sourceforge.net,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/2] kthread_worker: reimplement flush_kthread_work() to
 allow freeing the work item being executed

On Sun, 2012-07-22 at 09:49 -0700, Tejun Heo wrote:
> Hello,
> 
> On Sat, Jul 21, 2012 at 02:20:06PM -0400, Andy Walls wrote:
> > > +	worker->current_work = work;
> > >  	spin_unlock_irq(&worker->lock);
> > >  
> > >  	if (work) {
> > >  		__set_current_state(TASK_RUNNING);
> > >  		work->func(work);
> > 
> > If the call to 'work->func(work);' frees the memory pointed to by
> > 'work', 'worker->current_work' points to deallocated memory.
> > So 'worker->current_work' will only ever used as a unique 'work'
> > identifier to handle, correct?
> 
> Yeah.  flush_kthread_work(@work) which can only be called if @work is
> known to be alive looks at the pointer to determine whether it's the
> current work item on the worker.

OK.  Thanks.

Hmmm, I didn't know about the constraint about 'known to be alive' in
the other email I just sent.

That might make calling flush_kthread_work() hard for a user to use, if
the user lets the work get freed by another thread executing the work.


> > >  void flush_kthread_work(struct kthread_work *work)
> > >  {
> > > -	int seq = work->queue_seq;
> > > +	struct kthread_flush_work fwork = {
> > > +		KTHREAD_WORK_INIT(fwork.work, kthread_flush_work_fn),
> > > +		COMPLETION_INITIALIZER_ONSTACK(fwork.done),
> > > +	};
> > > +	struct kthread_worker *worker;
> > > +	bool noop = false;
> > > +
> > 
> > You might want a check for 'work == NULL' here, to gracefully handle
> > code like the following:

> workqueue's flush_work() doesn't allow %NULL pointer.  I don't want to
> make the behaviors deviate and don't see much point in changing
> workqueue's behavior at this point.

OK.  Fair enough.

Thanks.

Regards,
Andy


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