Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

[Oleg Nesterov <oleg@redhat.com>]

I continue to play the advocatus diaboli ;)

On 11/15, Mike Galbraith wrote:
>
> +static inline bool
> +task_wants_autogroup(struct task_struct *p, struct task_group *tg)
> +{
> +	if (tg != &root_task_group)
> +		return false;
> +
> +	if (p->sched_class != &fair_sched_class)
> +		return false;
> +
> +	if (p->flags & PF_EXITING)
> +		return false;

Hmm, why? Perhaps PF_EXITING was needed in the previous version to
avoid the race with release_task(). But now it is always safe to
use signal->autogroup.

And the exiting task can do a lot before it disappears, probably
we shouldn't ignore ->autogroup.

> +static void
> +autogroup_move_group(struct task_struct *p, struct autogroup *ag)
> +{
> +	struct autogroup *prev;
> +	struct task_struct *t;
> +	struct rq *rq;
> +	unsigned long flags;
> +
> +	rq = task_rq_lock(p, &flags);
> +	prev = p->signal->autogroup;
> +	if (prev == ag) {
> +		task_rq_unlock(rq, &flags);
> +		return;
> +	}
> +
> +	p->signal->autogroup = autogroup_kref_get(ag);
> +	__sched_move_task(p, rq);
> +	task_rq_unlock(rq, &flags);
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(t, &p->thread_group, thread_group) {
> +		sched_move_task(t);
> +	}
> +	rcu_read_unlock();

Not sure I understand why do we need rq->lock...

It can't protect the change of signal->autogroup, multiple callers
can use different rq's.

However. Currently the only callers holds ->siglock, so we are safe.
Perhaps we should just document that autogroup_move_group() needs
->siglock.

This also mean the patch can be simplified even more, __sched_move_task()
is not needed.

> +void sched_autogroup_fork(struct signal_struct *sig)
> +{
> +	sig->autogroup = autogroup_kref_get(current->signal->autogroup);
> +}

Well, in theory this can race with another thread doing autogroup_move_group().
We can read the old ->autogroup, and then use it after it was already freed.

Probably this needs ->siglock too.

Oleg.