[Devel] Re: [PATCH -mmotm 1/4] cgroups: Fix race between userspace and kernelspace

[KAMEZAWA Hiroyuki]

On Sat, 20 Feb 2010 00:28:16 +0200
"Kirill A. Shutemov" <kirill at shutemov.name> wrote:

> Notify userspace about cgroup removing only after rmdir of cgroup
> directory to avoid race between userspace and kernelspace.
> 
> Signed-off-by: Kirill A. Shutemov <kirill at shutemov.name>

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>

Hmm, but could you elaborate what is the race in patch description.
I can imagine by reading your patch but please write it in clear way.



> ---
>  kernel/cgroup.c |   32 +++++++++++++++++---------------
>  1 files changed, 17 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index ce9008f..46903cb 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -780,28 +780,15 @@ static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
>  static int cgroup_call_pre_destroy(struct cgroup *cgrp)
>  {
>  	struct cgroup_subsys *ss;
> -	struct cgroup_event *event, *tmp;
>  	int ret = 0;
>  
>  	for_each_subsys(cgrp->root, ss)
>  		if (ss->pre_destroy) {
>  			ret = ss->pre_destroy(ss, cgrp);
>  			if (ret)
> -				goto out;
> +				break;
>  		}
>  
> -	/*
> -	 * Unregister events and notify userspace.
> -	 */
> -	spin_lock(&cgrp->event_list_lock);
> -	list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
> -		list_del(&event->list);
> -		eventfd_signal(event->eventfd, 1);
> -		schedule_work(&event->remove);
> -	}
> -	spin_unlock(&cgrp->event_list_lock);
> -
> -out:
>  	return ret;
>  }
>  
> @@ -2991,7 +2978,6 @@ static void cgroup_event_remove(struct work_struct *work)
>  	event->cft->unregister_event(cgrp, event->cft, event->eventfd);
>  
>  	eventfd_ctx_put(event->eventfd);
> -	remove_wait_queue(event->wqh, &event->wait);
>  	kfree(event);
>  }
>  
> @@ -3009,6 +2995,7 @@ static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
>  	unsigned long flags = (unsigned long)key;
>  
>  	if (flags & POLLHUP) {
> +		remove_wait_queue_locked(event->wqh, &event->wait);
>  		spin_lock(&cgrp->event_list_lock);
>  		list_del(&event->list);
>  		spin_unlock(&cgrp->event_list_lock);
> @@ -3457,6 +3444,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
>  	struct dentry *d;
>  	struct cgroup *parent;
>  	DEFINE_WAIT(wait);
> +	struct cgroup_event *event, *tmp;
>  	int ret;
>  
>  	/* the vfs holds both inode->i_mutex already */
> @@ -3540,6 +3528,20 @@ again:
>  	set_bit(CGRP_RELEASABLE, &parent->flags);
>  	check_for_release(parent);
>  
> +	/*
> +	 * Unregister events and notify userspace.
> +	 * Notify userspace about cgroup removing only after rmdir of cgroup
> +	 * directory to avoid race between userspace and kernelspace
> +	 */
> +	spin_lock(&cgrp->event_list_lock);
> +	list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
> +		list_del(&event->list);
> +		remove_wait_queue(event->wqh, &event->wait);
> +		eventfd_signal(event->eventfd, 1);
> +		schedule_work(&event->remove);
> +	}
> +	spin_unlock(&cgrp->event_list_lock);
> +
>  	mutex_unlock(&cgroup_mutex);
>  	return 0;
>  }
> -- 
> 1.6.6.2
> 
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