Re: [PATCH v4] sched: automated per session task groups

Mike Galbraith
2010-11-21T16:36:09Z

On Sun, 2010-11-21 at 16:44 +0100, Oleg Nesterov wrote:
> On 11/21, Ingo Molnar wrote:
> >
> > Btw., there's a small cleanup in the patch that i picked up (see below), and i also
> > edited the commit log a bit - so you might want to pick up the version below.
>
> I didn't read the patch in details, but a couple of nits...
>
> > +static void
> > +autogroup_move_group(struct task_struct *p, struct autogroup *ag)
> > +{
> > + struct autogroup *prev;
> > + struct task_struct *t;
> > +
> > + spin_lock(&p->sighand->siglock);
>
> This needs spin_lock_irq(), ->siglock is irq-safe.

Ok.

> The same for other lockers, but:
>
> > +static inline struct autogroup *autogroup_get(struct task_struct *p)
> > +{
> > + struct autogroup *ag;
> > +
> > + /* task may be moved after we unlock.. tough */
> > + spin_lock(&p->sighand->siglock);
>
> This is called by fs/proc. In this case nothing protects us from
> release_task(), we can hit ->siglock == NULL (or we can race with
> exec which changes ->sighand in theory).

Oh my. Thanks.

(gad, signal_struct is sooo much harder to get right)

> This needs lock_task_sighand() (it can fail). Perhaps something
> else have the same problem...
>
> If the task is current and it is not exiting, or it is the new
> child (sched_autogroup_fork), then it is safe to use ->siglock
> directly.

Ok,

> And a pure cosmetic nit,
>
> > +void sched_autogroup_fork(struct signal_struct *sig)
> > +{
> > + struct sighand_struct *sighand = current->sighand;
> > +
> > + spin_lock(&sighand->siglock);
> > + sig->autogroup = autogroup_kref_get(current->signal->autogroup);
> > + spin_unlock(&sighand->siglock);
> > +}
>
> This looks a bit confusing. We do not need current->sighand->siglock
> to set sig->autogroup. Nobody except us can see this new signal_struct,
> and in any case current->sighand->siglock can't help.
>
> It is needed for autogroup_kref_get(), but we already have autogroup_get().
> I'd suggest
>
> void sched_autogroup_fork(struct signal_struct *sig)
> {
> sig->autogroup = autogroup_get(current);
> }

I'll do that.. once the thing is non-toxic to tip.

Thanks a lot Oleg.

-Mike