[RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork()

[Frederic Weisbecker]

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 at gmail.com>
> > Cc: Li Zefan <lizf at cn.fujitsu.com>
> > Cc: Tejun Heo <tj at kernel.org>
> > Cc: Containers <containers at lists.linux-foundation.org>
> > Cc: Cgroups <cgroups at vger.kernel.org>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> > Cc: Oleg Nesterov <oleg at redhat.com>
> > Cc: Andrew Morton <akpm at linux-foundation.org>
> > Cc: Paul Menage <paul at paulmenage.org>
> > Cc: Mandeep Singh Baines <msb at chromium.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);
> >  	}
> >  }