From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752541Ab1K1Nie (ORCPT ); Mon, 28 Nov 2011 08:38:34 -0500 Received: from mail-fx0-f46.google.com ([209.85.161.46]:56762 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751774Ab1K1Nid (ORCPT ); Mon, 28 Nov 2011 08:38:33 -0500 Date: Mon, 28 Nov 2011 17:38:28 +0400 From: Cyrill Gorcunov To: Andrey Vagin Cc: LKML , Li Zefan , Matt Helsley , Pavel Emelyanov , Tejun Heo Subject: Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup Message-ID: <20111128133828.GL1775@moon> References: <20111128120813.GK1775@moon> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?