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Date: Wed, 2 Jan 2019 08:16:40 -0800
From: Tejun Heo <tj@kernel.org>
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Zefan Li <lizefan@huawei.com>,
        syzbot <syzbot+87b93137e0280beaeba1@syzkaller.appspotmail.com>,
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Subject: Re: WARNING: lock held when returning to user space in grab_super
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Happy new year, Tetsuo.

On Wed, Jan 02, 2019 at 09:08:56PM +0900, Tetsuo Handa wrote:
> According to commit 633feee310de6b6c ("cgroup: refactor mount path and
> clearly distinguish v1 and v2 paths"), cgroup_do_mount() is failing to
> do full teardown steps for kernfs_mount() (deactivate_locked_super() ?)
> when kernfs_node_dentry() failed.

Hmm... that's basically dget()'ing the root dentry of the sb.  I'm not
sure how that could fail.  Can it?

Thanks.

-- 
tejun