linux-kernel - Re: [RFC PATCH] cgroup: Remove task_lock() from cgroup_post

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Thu, 22 Dec 2011 10:33:54 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Cgroups <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Menage <paul@...lmenage.org>,
	Mandeep Singh Baines <msb@...omium.org>
Subject: Re: [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork()

On Thu, Dec 22, 2011 at 04:55:49PM +0800, Li Zefan wrote:
> Frederic Weisbecker wrote:
> > cgroup_post_fork() is protected between threadgroup_change_begin()
> > and threadgroup_change_end() against concurrent changes of the
> > child's css_set in cgroup_task_migrate(). Also the child can't
> > exit and call cgroup_exit() at this stage, this means it's css_set
> > can't be changed with init_css_set concurrently.
> > 
> > For these reasons, we don't need to hold task_lock() on the child
> > because it's css_set can only remain stable in this place.
> > 
> > Let's remove the lock there.
> > 
> > NOTE: We could do something else: move threadgroup_change_end()
> > before cgroup_post_fork() and keep the task_lock() which, combined
> > with the css_set_lock, would be enough to synchronize against
> > cgroup_task_migrate()'s change on child->cgroup and its cglist.
> > Because outside that, the threadgroup lock doesn't appear to be
> > needed on cgroup_post_fork().
> > 
> 
> To narrow the scope of the threadgroup lock? I think it's preferable to keep
> cgroup_post_fork() inside the lock, to make things simpler and we have
> the same lock rule for both cgroup_fork() and cgroup_post_fork().

Ok!
 
> > Let's debate!
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Li Zefan <lizf@...fujitsu.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Containers <containers@...ts.linux-foundation.org>
> > Cc: Cgroups <cgroups@...r.kernel.org>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Paul Menage <paul@...lmenage.org>
> > Cc: Mandeep Singh Baines <msb@...omium.org>
> > ---
> >  kernel/cgroup.c |   11 ++++++++---
> >  1 files changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> > index 4936d88..d0dbf72 100644
> > --- a/kernel/cgroup.c
> > +++ b/kernel/cgroup.c
> > @@ -4622,10 +4622,15 @@ void cgroup_post_fork(struct task_struct *child)
> >  {
> >  	if (use_task_css_set_links) {
> >  		write_lock(&css_set_lock);
> > -		task_lock(child);
> > -		if (list_empty(&child->cg_list))
> > +		if (list_empty(&child->cg_list)) {
> > +			/*
> > +			 * It's safe to use child->cgroups without task_lock()
> > +			 * here because we are protected through
> > +			 * threadgroup_change_begin() against concurrent
> > +			 * css_set change in cgroup_task_migrate()
> > +			 */
> 
> Also explain why it won't race with cgroup_exit()? You were not quite confident
> about that before Oleg's explanation. ;)

hehe indeed :)

Will update, thanks!

> >  			list_add(&child->cg_list, &child->cgroups->tasks);
> > -		task_unlock(child);
> > +		}
> >  		write_unlock(&css_set_lock);
> >  	}
> >  }
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/