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

From: Paul E. McKenney
Date: Thu Jul 31 2014 - 13:02:53 EST

On Thu, Jul 31, 2014 at 06:31:38PM +0200, Oleg Nesterov wrote:
> 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() ?

If I add the list_del_rcu() there, then I am back to a concurrent list,
which I would like to avoid. Don't get me wrong, it was fun playing with
the list-locked stuff, but best to avoid it if we can.

Also, the current implementation implicitly locks down the task_structs
via the exit path. The nice thing about using get_task_struct to lock
them down is that -only- the task_struct itself is locked down -- the
task can be reaped and so on. In contrast, the current exit_rcu_tasks()
approach delays the semantic exit instead of just hanging onto the
underlying task_struct a bit longer.

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

I am worried about a task that does a voluntary context switch, then exits.
This could results in rcu_tasks_kthread() and __unhash_process() both
wanting to dequeue at the same time, right?

Thanx, Paul

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