linux-kernel - Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup

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Date:	Mon, 28 Nov 2011 18:03:56 +0400
From:	Andrew Vagin <avagin@...nvz.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrey Vagin <avagin@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Matt Helsley <matthltc@...ibm.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen
 cgroup

> > > +static int freezer_can_attach_task(struct cgroup *cgroup, struct task_struct *task)
> > > +{
> > > +       struct freezer *old_freezer;
> > > +       struct freezer *freezer;
> > > +
> > > +       int goal_state, orig_state;
> > > +       int retval = 0;
> > > +
> > > +       old_freezer = task_freezer(task);
> > > +       freezer = cgroup_freezer(cgroup);
> > > +
> > > +       spin_lock_irq(&freezer->lock);
> > > +
> > > +       if (!spin_trylock_irq(&old_freezer->lock)) {
> > > +               retval = -EBUSY;
> > 
> > I think EBUSY is not a good idea in this place.  We can do something
> > like double_rq_lock.
> > 
> 
> Could you please elaborate? freezers are guarded with spinlocks so I think
> we should stick with them instead of poking rq (or whatever) directly.

It's misunderstanding. I want to say that we can avoid dead lock if we
will take a lock with a smaller address at first.

if (&freezer->lock > &old_freezer->lock) {
	spin_lock_irq(&old_freezer->lock)
	spin_lock_irq(&freezer->lock);
} else {
	spin_lock_irq(&freezer->lock);
	spin_lock_irq(&old_freezer->lock)
}

> 
> > 
> > > +
> > > +static void freezer_cancel_attach(struct cgroup_subsys *ss,
> > > +                                 struct cgroup *cgroup,
> > > +                                 struct cgroup *old_cgroup,
> > > +                                 struct task_struct *task)
> > > +{
> > > +       struct freezer *freezer = cgroup_freezer(old_cgroup);
> > > +       int retval = 0;
> > > +
> > > +       spin_lock_irq(&freezer->lock);
> > > +       retval = freezer_task_transition(task, freezer->state);
> > > +       if (retval)
> > > +               pr_warning("freezer: Can't move task (pid %d) to %s state\n",
> > > +                          task_pid_nr(task),
> > > +                          freezer_state_strs[freezer->state]);
> > 
> > It's strange. A rollback can't fail. We have three situations:
> > 
> > frozen -> frozen
> > thawed -> frozen
> > frozen -> thawed
> > 
> > In first and second cases cancel_request can't fail.
> > In the third we have a problem, which may be solved if we will call
> > thaw_process(task) from attach_task(), we can do that, because
> > thaw_process() can't fail. It solves a problem, because
> > freezer_cancel_attach will be executed for the first and second cases
> > only.
> > 
> > If my suggestion is correct, we can replace pr_warning on BUG_ON
> > 
> 
> Yes, the case which can fail is
> 
>    frozen->(can_attach_task)->thawed
>      (cgroup_task_migrate failure)
>    thawed->(cancel_attach)->frozen
> 
> and we should never fail here since otherwise we would not have
> a "frozen" state before. But I think placing BUG_ON is too severe
> here, maybe WARN_ON_ONCE(1) would fit better?

It's true, if a task is not being executed between thaw_process() and
freeze_task().
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