linux-kernel - Re: [PATCH 2/2] cgroup: fix cgroup hierarchy umount race

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Message-ID: <20120714120852.GK22927@ZenIV.linux.org.uk>
Date:	Sat, 14 Jul 2012 13:08:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	'Tejun Heo' <tj@...nel.org>
Cc:	Shyju PV <shyju.pv@...wei.com>, 'Li Zefan' <lizefan@...wei.com>,
	'Sanil kumar' <sanil.kumar@...wei.com>,
	'Masanari Iida' <standby24x7@...il.com>,
	'LKML' <linux-kernel@...r.kernel.org>,
	'Cgroups' <cgroups@...r.kernel.org>, levinsasha928@...il.com,
	nagamani.mantha@...wei.com
Subject: Re: [PATCH 2/2] cgroup: fix cgroup hierarchy umount race

On Sat, Jul 07, 2012 at 04:46:59PM -0700, 'Tejun Heo' wrote:
> Fix it by holding an extra superblock->s_active reference across
> dput() from css release, which is the dput() path added by 48ddbe1946
> and the only one which doesn't hold an extra s_active ref across the
> final cgroup dput().

> @@ -3883,8 +3883,12 @@ static void css_dput_fn(struct work_struct *work)
>  {
>  	struct cgroup_subsys_state *css =
>  		container_of(work, struct cgroup_subsys_state, dput_work);
> +	struct dentry *dentry = css->cgroup->dentry;
> +	struct super_block *sb = dentry->d_sb;
>  
> -	dput(css->cgroup->dentry);
> +	atomic_inc(&sb->s_active);
> +	dput(dentry);
> +	deactivate_super(sb);
>  }

While we are at it, what guarantees that css->dput_work will complete before
css->cgroup or the object containing css get freed under us?
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