Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks()

[Oleg Nesterov]

Again, sorry, I didn't read the patches yet, just noticed your discussion...

On 07/31, Paul E. McKenney wrote:
>
> On Thu, Jul 31, 2014 at 03:30:12PM +0800, Lai Jiangshan wrote:
>
> > > +				t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> > > +				t->rcu_tasks_holdout = 1;
> > > +				list_add(&t->rcu_tasks_holdout_list,
> > > +					 &rcu_tasks_holdouts);
> >
> > I think get_task_struct() is needed here to avoid the task disappears.
>
> Hmmm...  Let's see...
>
> Looks like get_task_struct() does a blind atomic increment of ->usage.
> And put_task_struct() does an atomic_dec_and_test().  So one question
> is "what prevents us from doing get_task_struct() after the final
> put_task_struct() has pushed ->usage down to zero?"
>
> Hopefully there is a grace period in there somewhere, otherwise it will
> be necessary to take the task-list lock, which I would like to avoid.
>
> Looks like the call_rcu() of delayed_put_task_struct() in release_task()
> might be doing this.

Yes, exactly, so get_task_struct() is always fine as long as task_struct
itself is protected by RCU.

But can't we avoid get_task_struct()? This can pin a lot of task_struct's.
Can't we just add list_del_rcu(holdout_list) into __unhash_process() ?

We only need to ensure that list_add() above can't race with that list_del(),
perhaps we can tolerate lock_task_sighand() ?

Oleg.

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