linux-kernel - Re: [PATCH] cgroup: fix dentry still in use bug when dropping css refs after umount

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Message-ID: <20120716164527.GB30872@google.com>
Date:	Mon, 16 Jul 2012 09:45:27 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	shyju pv <shyju.pv@...wei.com>,
	Sanil kumar <sanil.kumar@...wei.com>,
	Masanari Iida <standby24x7@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, viro@...iv.linux.org.uk,
	levinsasha928@...il.com
Subject: Re: [PATCH] cgroup: fix dentry still in use bug when dropping css
 refs after umount

Hello,

On Tue, Jul 10, 2012 at 10:11:05AM +0800, Li Zefan wrote:
> Right, but that should be safe. The css objects of the root cgroup are
> allocated at boot, and won't be destroyed at umount.
> 
> Furthermore when a cgroup hierarchy is going to be unmounted, those css's
> will be made to point to a cgroup named dummytop in rebind_subsystems(),
> and there's a syncronize_rcu() in the end of the function, so accessing
> css->cgroup is always safe.
> 
> In this case, dummytop->dentry is NULL, and that's safe too, because
> cgroup_path() is aware of this case.

I see.  Thanks for the explanation.

-- 
tejun
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