Messages in this thread | | | Date | Mon, 28 Nov 2011 19:00:54 +0400 | From | Cyrill Gorcunov <> | Subject | Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup |
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On Mon, Nov 28, 2011 at 06:03:56PM +0400, Andrew Vagin wrote: > > > > +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) > } >
This is not applicable here as far as I see. It works for rq because of per-cpu address allocation, but not for freezers which are allocated via kzalloc. The second try_lock (note I've overdid with irq disabling, plain spin_trylock would be enough) is not for escaping deadlock but rather for not waiting much if target freezer is handling state transition for all task it has.
I think the better approach would to make this code even less lock contended, ie something like
local_irq_disable spin_trylock(new_freezer) spin_trylock(old_freezer) ... local_irq_enable
so if both freezers are not handling anything we attach the task then. Or I miss something obvious?
> > > > > > 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().
Hmm... But what the problem it might be if a task get executed between those stages even for some time? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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