Re: lockdep trace from posix timers

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2012-08-20 at 17:05 +0200, Oleg Nesterov wrote:
> On 08/20, Peter Zijlstra wrote:
> >
> > +static void __task_work_run(struct callback_head *tail)
> >  {
> > -	struct task_struct *task = current;
> > -	struct callback_head *p, *q;
> > -
> > -	while (1) {
> > -		raw_spin_lock_irq(&task->pi_lock);
> > -		p = task->task_works;
> > -		task->task_works = NULL;
> > -		raw_spin_unlock_irq(&task->pi_lock);
> > -
> > -		if (unlikely(!p))
> > -			return;
> > -
> > -		q = p->next; /* head */
> > -		p->next = NULL; /* cut it */
> > -		while (q) {
> > -			p = q->next;
> > -			q->func(q);
> > -			q = p;
> > +	struct callback_head **head = &current->task_works;
> > +
> > +	do {
> > +		struct callback_head *work = xchg(head, NULL);
> > +		while (work) {
> > +			struct callback_head *next = ACCESS_ONCE(work->next);
> > +
> > +			WARN_ON_ONCE(work == &dead);
> > +
> > +			work->func(work);
> > +			work = next;
> >  		}
> > -	}
> > +	} while (cmpxchg(head, NULL, tail) != NULL);
> 
> Yes, we can add the explicit argument to __task_work_run(), but it can
> check PF_EXITING instead, this looks simpler to me.

I guess we could.. but I thought the explicit callback was simpler ;-)

> Note also your patch breaks fifo, but this is fixable.

Why do you care about the order? Iterating a single linked queue in fifo
seems more expensive than useful.