Messages in this thread | | | Date | Mon, 28 Nov 2011 17:38:28 +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 05:10:00PM +0400, Andrey Vagin wrote: > > void (*cancel_attach)(struct cgroup_subsys *ss, struct cgroup *cgrp, > > - struct task_struct *tsk); > > + struct cgroup *old_cgrp, struct task_struct *tsk); > > I'm not shure, that we need old_cgrp, because when cancel_attach is > executed, a task is in old cgroup and old_cgrp = task_cgroup(tsk); > > ... >
Yup, thanks for the point. Indeed old_cgrp is redundant and task_cgroup helper will provide all additional information we need.
> > + > > +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.
> > > + > > +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? -- 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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