linux-kernel - Re: [PATCH v21 015/100] cgroup freezer: Update stale locking comments

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Message-ID: <20100506203117.GM31830@count0.beaverton.ibm.com>
Date:	Thu, 6 May 2010 13:31:17 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Oren Laadan <orenl@...columbia.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, Serge Hallyn <serue@...ibm.com>,
	Matt Helsley <matthltc@...ibm.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Cedric Le Goater <legoater@...e.fr>,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH v21 015/100] cgroup freezer: Update stale locking
 comments

On Thu, May 06, 2010 at 09:40:19PM +0200, Rafael J. Wysocki wrote:
> On Saturday 01 May 2010, Oren Laadan wrote:
> > From: Matt Helsley <matthltc@...ibm.com>
> > 
> > Update stale comments regarding locking order and add a little more detail
> > so it's easier to follow the locking between the cgroup freezer and the
> > power management freezer code.
> 
> I guess these three patches are for me.
> 
> Do you want me to handle them?
> 
> Rafael

Hi Rafael,

	I think you can take 15 as it's an update to the comments.

16 and 17 are specific to checkpoint/restart. There are no other in-kernel
uses, that we've identified, for them. So I was under the impression that it's
inappropriate to add them without also adding the checkpoint/restart patches
that use them.

Thanks!

Cheers,
	-Matt Helsley

> 
> 
> > Signed-off-by: Matt Helsley <matthltc@...ibm.com>
> > Cc: Oren Laadan <orenl@...columbia.edu>
> > Cc: Cedric Le Goater <legoater@...e.fr>
> > Cc: Paul Menage <menage@...gle.com>
> > Cc: Li Zefan <lizf@...fujitsu.com>
> > Cc: Rafael J. Wysocki <rjw@...k.pl>
> > Cc: linux-pm@...ts.linux-foundation.org
> > ---
> >  kernel/cgroup_freezer.c |   21 +++++++++++++--------
> >  1 files changed, 13 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> > index da5e139..85f19cc 100644
> > --- a/kernel/cgroup_freezer.c
> > +++ b/kernel/cgroup_freezer.c
> > @@ -89,10 +89,10 @@ struct cgroup_subsys freezer_subsys;
> >  
> >  /* Locks taken and their ordering
> >   * ------------------------------
> > - * css_set_lock
> >   * cgroup_mutex (AKA cgroup_lock)
> > - * task->alloc_lock (AKA task_lock)
> >   * freezer->lock
> > + * css_set_lock
> > + * task->alloc_lock (AKA task_lock)
> >   * task->sighand->siglock
> >   *
> >   * cgroup code forces css_set_lock to be taken before task->alloc_lock
> > @@ -100,33 +100,38 @@ struct cgroup_subsys freezer_subsys;
> >   * freezer_create(), freezer_destroy():
> >   * cgroup_mutex [ by cgroup core ]
> >   *
> > - * can_attach():
> > - * cgroup_mutex
> > + * freezer_can_attach():
> > + * cgroup_mutex (held by caller of can_attach)
> >   *
> > - * cgroup_frozen():
> > + * cgroup_freezing_or_frozen():
> >   * task->alloc_lock (to get task's cgroup)
> >   *
> >   * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
> > - * task->alloc_lock (to get task's cgroup)
> >   * freezer->lock
> >   *  sighand->siglock (if the cgroup is freezing)
> >   *
> >   * freezer_read():
> >   * cgroup_mutex
> >   *  freezer->lock
> > + *   write_lock css_set_lock (cgroup iterator start)
> > + *    task->alloc_lock
> >   *   read_lock css_set_lock (cgroup iterator start)
> >   *
> >   * freezer_write() (freeze):
> >   * cgroup_mutex
> >   *  freezer->lock
> > + *   write_lock css_set_lock (cgroup iterator start)
> > + *    task->alloc_lock
> >   *   read_lock css_set_lock (cgroup iterator start)
> > - *    sighand->siglock
> > + *    sighand->siglock (fake signal delivery inside freeze_task())
> >   *
> >   * freezer_write() (unfreeze):
> >   * cgroup_mutex
> >   *  freezer->lock
> > + *   write_lock css_set_lock (cgroup iterator start)
> > + *    task->alloc_lock
> >   *   read_lock css_set_lock (cgroup iterator start)
> > - *    task->alloc_lock (to prevent races with freeze_task())
> > + *    task->alloc_lock (inside thaw_process(), prevents race with refrigerator())
> >   *     sighand->siglock
> >   */
> >  static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
> > 
> 
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