linux-kernel - Re: [PATCH] cgroup: Fix potential race between cgroup

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Message-ID: <20170831011230.GD491396@devbig577.frc2.facebook.com>
Date:   Wed, 30 Aug 2017 18:12:30 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     lizefan@...wei.com, mingo@...nel.org, longman@...hat.com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        sramana@...eaurora.org, prsood@...eaurora.org
Subject: Re: [PATCH] cgroup: Fix potential race between cgroup_exit and
 migrate path

On Wed, Aug 30, 2017 at 06:03:19PM -0700, Tejun Heo wrote:
> Oops, more like the following.
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index df2e0f1..6f34025 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -683,7 +683,7 @@ static void css_set_move_task(struct task_struct *task,
>  			if (it->task_pos == &task->cg_list)
>  				css_task_iter_advance(it);
>  
> -		list_del_init(&task->cg_list);
> +		list_del(&task->cg_list);
>  		if (!css_set_populated(from_cset))
>  			css_set_update_populated(from_cset, false);
>  	} else {
> @@ -702,6 +702,8 @@ static void css_set_move_task(struct task_struct *task,
>  		rcu_assign_pointer(task->cgroups, to_cset);
>  		list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
>  							     &to_cset->tasks);
> +	} else {
> +		INIT_LIST_HEAD(&task->cg_list);
>  	}
>  }

On the third thought, I don't think this can happen either because now
migration is strongly synchronized against exits.  Please take a look
at the changes around cgroup_threadgroup_rwsem.

Thanks.

-- 
tejun