Re: [PATCH v2] cgroup: remove redundate get/put of old css_set from migrate

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Dec 19, 2011 at 10:44:34AM -0800, Mandeep Singh Baines wrote:
> Changes in V2:
> * Updated commit message as per Tejun's feedback:
>   * https://lkml.org/lkml/2011/12/19/289
> 
> -- >8 -- (snip)
> 
> We can now assume that the css_set reference held by the task
> will not go away for an exiting task. PF_EXITING state can be
> trusted throughout migration by checking it after locking
> threadgroup.
> 
> While at it, renamed css_set_check_fetched to css_set_fetched.
> !css_set_fetched() seems to read better than
> !css_set_check_fetched().
> 
> This patch depends on:
> 
> commit cd3d095275374220921fcf0d4e0c16584b26ddbc
> Author: Tejun Heo <tj@kernel.org>
> Date:   Mon Dec 12 18:12:21 2011 -0800
> 
>     cgroup: always lock threadgroup during migration
> 
> Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: containers@lists.linux-foundation.org
> Cc: cgroups@vger.kernel.org
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> ---
>  kernel/cgroup.c |   25 ++++++-------------------
>  1 files changed, 6 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1b3b841..eb95e32 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1856,7 +1856,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  	 */
>  	task_lock(tsk);
>  	oldcg = tsk->cgroups;
> -	get_css_set(oldcg);
>  	task_unlock(tsk);
>  
>  	/* locate or allocate a new css_set for this task. */
> @@ -1872,12 +1871,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  		might_sleep();
>  		/* find_css_set will give us newcg already referenced. */
>  		newcg = find_css_set(oldcg, cgrp);
> -		if (!newcg) {
> -			put_css_set(oldcg);
> +		if (!newcg)
>  			return -ENOMEM;
> -		}
>  	}
> -	put_css_set(oldcg);
>  
>  	/* @tsk can't exit as its threadgroup is locked */
>  	task_lock(tsk);
> @@ -2015,9 +2011,8 @@ struct cg_list_entry {
>  	struct list_head links;
>  };
>  
> -static bool css_set_check_fetched(struct cgroup *cgrp,
> -				  struct task_struct *tsk, struct css_set *cg,
> -				  struct list_head *newcg_list)
> +static bool css_set_fetched(struct cgroup *cgrp, struct task_struct *tsk,
> +			    struct css_set *cg, struct list_head *newcg_list)
>  {
>  	struct css_set *newcg;
>  	struct cg_list_entry *cg_entry;
> @@ -2191,19 +2186,11 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>  		/* get old css_set pointer */
>  		task_lock(tc->task);
>  		oldcg = tc->task->cgroups;
> -		get_css_set(oldcg);
>  		task_unlock(tc->task);
> -		/* see if the new one for us is already in the list? */
> -		if (css_set_check_fetched(cgrp, tc->task, oldcg, &newcg_list)) {
> -			/* was already there, nothing to do. */
> -			put_css_set(oldcg);
> -		} else {
> -			/* we don't already have it. get new one. */
> -			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
> -			put_css_set(oldcg);
> -			if (retval)
> +		/* if we don't already have it in the list get a new one */
> +		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
> +			if (css_set_prefetch(cgrp, oldcg, &newcg_list))

Forgot to put the error value in retval?

>  				goto out_list_teardown;
> -		}
>  	}
>  
>  	/*
> -- 
> 1.7.3.1
> 

Thanks.