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

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Date:	Fri, 26 Feb 2016 16:38:18 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kbuild test robot <lkp@...el.com>, kbuild-all@...org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	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 v5 08/20] kthread: Allow to cancel kthread work

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);
> >  
> 
> This is pretty vile; why can't you drop the lock over del_timer_sync() ?

We would need to take the lock later and check if nobody has set the timer
again in the meantime.

Now, timer_active() check is not reliable. It does not check if the
timer handler is running at the moment. I tried to implement
a safe timer_active()[1] but nobody was keen to take it.

Even if we have that timer_active() check, we would need to add
some relock/check/try_again stuff around the del_timer_sync().
So, it would just move the complexity somewhere else.

I think that the current solution is quite elegant after all.
Thanks a lot Tejun for the idea.

Reference:
[1] http://thread.gmane.org/gmane.linux.kernel.mm/144964/focus=144965


Thanks a lot for review,
Petr