Re: [PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()

[Peter Zijlstra <peterz@infradead.org>]

On Sun, 2011-06-05 at 21:12 +0200, Oleg Nesterov wrote:
> On 06/03, Peter Zijlstra wrote:
> >
> > @@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p,
> >  			!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
> >
> >  #ifdef CONFIG_LOCKDEP
> > +	/*
> > +	 * The caller should hold either p->pi_lock or rq->lock, when changing
> > +	 * a task's CPU.
> 
> Is it literally true? IIRC, we need ->pi_lock if the task is not active,
> and rq->lock if p->on_rq = 1. And that is why we do not clear p->on_rq
> between deactivate_task() + activate_task(), correct?
> 
> > +	 *
> > +	 * sched_move_task() holds both and thus holding either pins the cgroup,
> > +	 * see set_task_rq().
> > +	 *
> > +	 * Furthermore, all task_rq users should acquire both locks, see
> > +	 * task_rq_lock().
> > +	 */
> >  	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
> >  				      lockdep_is_held(&task_rq(p)->lock)));
> 
> IOW, perhaps this should be
> 
> 	WARN_ON_ONCE(debug_locks && !lockdep_is_held(p->on_rq ?
> 					&task_rq(p)->lock : &p->pi_lock))
> 
> ?
> 
> Not that I really suggest to change this WARN_ON(), I am just trying
> to recall the new rules.

You're right, p->pi_lock for wakeups, rq->lock for runnable tasks.