Re: [PATCH 4/5] freezer cgroup: Fix an RCU warning in cgroup_freezing_or_frozen()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-04-22 at 17:31 +0800, Li Zefan wrote:
> with CONFIG_PROVE_RCU, a warning can be triggered when we
> resume from suspend:
> 
> ...
> include/linux/cgroup.h:533 invoked rcu_dereference_check() without protection!
> ...
> 
> task_freezer() calls task_subsys_state(), which needs to be
> protected by rcu_read_lock or cgroup_mutex.
> 
> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> ---
>  kernel/cgroup_freezer.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index 5038f4c..ac76983 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -53,6 +53,7 @@ int cgroup_freezing_or_frozen(struct task_struct *task)
>  	struct freezer *freezer;
>  	enum freezer_state state;
>  
> +	rcu_read_lock();
>  	task_lock(task);
>  	freezer = task_freezer(task);
>  	if (!freezer->css.cgroup->parent)
> @@ -60,6 +61,7 @@ int cgroup_freezing_or_frozen(struct task_struct *task)
>  	else
>  		state = freezer->state;
>  	task_unlock(task);
> +	rcu_read_unlock();
>  
>  	return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
>  }

Hmm cgroup_attach_task() does hold task_lock() over setting
tsk->cgroups, so doesn't that also pin the task to the cgroup and thus
the cgroup itself?