linux-kernel - Re: linux-next: cgroup_mount() falls asleep forever

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Message-ID: <CAM_iQpU4rZLxdb=1TkG6HMR54SLk7tmRqxkg0+acRtV91o+UXA@mail.gmail.com>
Date:	Wed, 24 Sep 2014 11:31:24 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Andrey Wagin <avagin@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: linux-next: cgroup_mount() falls asleep forever

On Wed, Sep 24, 2014 at 7:29 AM, Andrey Wagin <avagin@...il.com> wrote:
> 2014-09-24 14:31 GMT+04:00 Andrey Wagin <avagin@...il.com>:
>> Hi All,
>
> The problem is in a following commit:
>
> commit 0c7bf3e8cab7900e17ce7f97104c39927d835469
> Author: Zefan Li <lizefan@...wei.com>
> Date:   Sat Sep 20 14:49:10 2014 +0800
>
>     cgroup: remove redundant variable in cgroup_mount()
>
>     Both pinned_sb and new_sb indicate if a new superblock is needed,
>     so we can just remove new_sb.
>
>     Note now we must check if kernfs_tryget_sb() returns NULL, because
>     when it returns NULL, kernfs_mount() may still re-use an existing
>     superblock, which is just allocated by another concurent mount.
>

I guess the check for NULL is incorrect, the comment on kernfs_pin_sb()
says:

Returns NULL if there's no superblock associated to this kernfs_root,
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