Messages in this thread | | | Date | Mon, 28 Nov 2011 18:03:56 +0400 | From | Andrew Vagin <> | Subject | Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup |
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> > > +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(). -- 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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