Re: [PATCH v5 08/20] kthread: Allow to cancel kthread work

[Peter Zijlstra]

On Fri, Feb 26, 2016 at 06:00:56PM +0100, Petr Mladek wrote:
> On Fri 2016-02-26 17:25:52, Peter Zijlstra wrote:
> > On Fri, Feb 26, 2016 at 04:38:18PM +0100, Petr Mladek wrote:
> > > On Thu 2016-02-25 13:59:32, Peter Zijlstra wrote:
> > > > On Wed, Feb 24, 2016 at 05:18:05PM +0100, Petr Mladek wrote:
> > > > > @@ -770,7 +782,22 @@ void delayed_kthread_work_timer_fn(unsigned long __data)
> > > > >  	if (WARN_ON_ONCE(!worker))
> > > > >  		return;
> > > > >  
> > > > > -	spin_lock(&worker->lock);
> > > > > +	/*
> > > > > +	 * We might be unable to take the lock if someone is trying to
> > > > > +	 * cancel this work and calls del_timer_sync() when this callback
> > > > > +	 * has already been removed from the timer list.
> > > > > +	 */
> > > > > +	while (!spin_trylock(&worker->lock)) {
> > > > > +		/*
> > > > > +		 * Busy wait with spin_is_locked() to avoid cache bouncing.
> > > > > +		 * Break when canceling is set to avoid a deadlock.
> > > > > +		 */
> > > > > +		do {
> > > > > +			if (work->canceling)
> > > > > +				return;
> > > > > +			cpu_relax();
> > > > > +		} while (spin_is_locked(&worker->lock));
> > > > > +	}
> > > > >  	/* Work must not be used with more workers, see queue_kthread_work(). */
> > > > >  	WARN_ON_ONCE(work->worker != worker);

> > And since you do add_timer() while holding the spinlock, this should all
> > work out, no?
> 
> Interesting idea. Yes, it should work. But is this really easier? The
> try_again/relock/recheck code is not trivial either.

The trylock loop has very bad worst case behaviour, it really is best to
avoid such patterns if at all possible.