linux-kernel - Re: [PATCH 2/4] freezer_cg: remove redundant check in freezer_can

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Date:	Tue, 21 Oct 2008 09:32:50 +0200
From:	Cedric Le Goater <clg@...ibm.com>
To:	Li Zefan <lizf@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] freezer_cg: remove redundant check in freezer_can_attach()

Li Zefan wrote:
> It is sufficient to check if @task is frozen, and no need to check if
> the original freezer is frozen.

hmm, a task being frozen does not mean that its freezer cgroup is 
frozen. So the extra check seems valid but looking at the comment :

	/*
	 * The call to cgroup_lock() in the freezer.state write method prevents
	 * a write to that file racing against an attach, and hence the
	 * can_attach() result will remain valid until the attach completes.
	 */
	static int freezer_can_attach(struct cgroup_subsys *ss,

how do we know that the task_freezer(task), which is not protected by
the cgroup_lock(), is not going to change its state to CGROUP_FROZEN 
it looks racy.

C.

> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  kernel/cgroup_freezer.c |   16 +++++++---------
>  1 files changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index 7f54d1c..6fadafe 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -162,9 +162,13 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
>  			      struct task_struct *task)
>  {
>  	struct freezer *freezer;
> -	int retval;
>  
> -	/* Anything frozen can't move or be moved to/from */
> +	/*
> +	 * Anything frozen can't move or be moved to/from.
> +	 *
> +	 * Since orig_freezer->state == FROZEN means that @task has been
> +	 * frozen, so it's sufficient to check the latter condition.
> +	 */
>  
>  	if (is_task_frozen_enough(task))
>  		return -EBUSY;
> @@ -173,13 +177,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
>  	if (freezer->state == CGROUP_FROZEN)
>  		return -EBUSY;
>  
> -	retval = 0;
> -	task_lock(task);
> -	freezer = task_freezer(task);
> -	if (freezer->state == CGROUP_FROZEN)
> -		retval = -EBUSY;
> -	task_unlock(task);
> -	return retval;
> +	return 0;
>  }
>  
>  static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)

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