[Devel] Re: [RFC][PATCH] Container Freezer: Don't Let Frozen Stuff Change

[Matt Helsley]

On Thu, 2008-07-10 at 11:20 +0800, Li Zefan wrote:
> Matt Helsley wrote:
> > On Thu, 2008-07-10 at 09:42 +0900, KAMEZAWA Hiroyuki wrote:
> >> On Wed, 09 Jul 2008 14:58:43 -0700
> >> Matt Helsley <matthltc at us.ibm.com> wrote:
> >>
> >>> On Tue, 2008-07-08 at 13:07 -0700, Paul Menage wrote:
> >>>> On Tue, Jul 8, 2008 at 1:06 PM, Paul Menage <menage at google.com> wrote:
> >>>>> On Tue, Jul 8, 2008 at 12:39 PM, Matt Helsley <matthltc at us.ibm.com> wrote:
> >>>>>> One is to try and disallow users from moving frozen tasks. That doesn't
> >>>>>> seem like a good approach since it would require a new cgroups interface
> >>>>>> "can_detach()".
> >>>>> Detaching from the old cgroup happens at the same time as attaching to
> >>>>> the new cgroup, so can_attach() would work here.
> >>> Update: I've made a patch implementing this. However it might be better
> >>> to just modify attach() to thaw the moving task rather than disallow
> >>> moving the frozen task. Serge, Cedric, Kame-san, do you have any
> >>> thoughts on which is more useful and/or intuitive?
> >>>
> >> Thank you for explanation in previous mail.
> >>
> >> Hmm, just thawing seems atractive but it will confuse people (I think).
> >>
> >> I think some kind of process-group is freezed by this freezer and "moving
> >> freezed task" is wrong(unexpected) operation in general.  And there will
> >> be no demand to do that from users.
> >> I think just taking "moving freezed task" as error-operation and returning
> >> -EBUSY is better.
> > 
> > Kame-san,
> > 
> > 	I've been working on changes to the can_attach() code so it was pretty
> > easy to try this out.
> > 
> > 	Don't let frozen tasks or cgroups change. This means frozen tasks can't
> > leave their current cgroup for another cgroup. It also means that tasks
> > cannot be added to or removed from a cgroup in the FROZEN state. We
> > enforce these rules by checking for frozen tasks and cgroups in the
> > can_attach() function.
> > 
> > Signed-off-by: Matt Helsley <matthltc at us.ibm.com>
> > ---
> > Builds, boots, passes testing against 2.6.26-rc5-mm2
> > 
> >  kernel/cgroup_freezer.c |   42 +++++++++++++++++++++++++-----------------
> >  1 file changed, 25 insertions(+), 17 deletions(-)
> > 
> > Index: linux-2.6.26-rc5-mm2/kernel/cgroup_freezer.c
> > ===================================================================
> > --- linux-2.6.26-rc5-mm2.orig/kernel/cgroup_freezer.c
> > +++ linux-2.6.26-rc5-mm2/kernel/cgroup_freezer.c
> > @@ -89,26 +89,43 @@ static void freezer_destroy(struct cgrou
> >  			    struct cgroup *cgroup)
> >  {
> >  	kfree(cgroup_freezer(cgroup));
> >  }
> >  
> > +/* Task is frozen or will freeze immediately when next it gets woken */
> > +static bool is_task_frozen_enough(struct task_struct *task)
> > +{
> > +	return (frozen(task) || (task_is_stopped_or_traced(task) && freezing(task)));
> > +}
> >  
> > +/*
> > + * The call to cgroup_lock() in the freezer.state write method prevents
> > + * a write to that file racing against an attach, and hence the
> > + * can_attach() result will remain valid until the attach completes.
> > + */
> >  static int freezer_can_attach(struct cgroup_subsys *ss,
> >  			      struct cgroup *new_cgroup,
> >  			      struct task_struct *task)
> >  {
> >  	struct freezer *freezer;
> > -	int retval = 0;
> > +	int retval;
> > +
> > +	/* Anything frozen can't move or be moved to/from */
> > +
> > +	if (is_task_frozen_enough(task))
> > +		return -EBUSY;
> >  
> 
> cgroup_lock() can prevent the state change of old_cgroup and new_cgroup, but
> will the following racy happen ?
>    1                                     2

For most of the paths using these functions we have:

cgroup_lock()                              cgroup_lock()
...                                        ...
> can_attach(tsk)
>   is_task_frozen_enough(tsk) == false
>                                          freeze_task(tsk)
                                           or thaw_process(tsk)
> attach(tsk)
...                                        ...
cgroup_unlock()                            cgroup_unlock()

	I've checked the cgroup freezer subsystem and the cgroup "core" and
this interleaving isn't possible between those two pieces. Only the
swsusp invocation of freeze_task() does not protect freeze/thaw with the
cgroup_lock. I'll be looking into this some more to see if that's really
a problem and if so how we might solve it.

	Thanks for this excellent question.

Cheers,
	-Matt Helsley

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