linux-kernel - Re: [PATCH] cgroup: fix top cgroup refcnt leak

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Date:	Fri, 14 Feb 2014 19:15:18 +0800
From:	Li Zefan <lizf.kern@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Li Zefan <lizefan@...wei.com>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH] cgroup: fix top cgroup refcnt leak

于 2014年02月14日 17:36, Li Zefan 写道:
> If we mount the same cgroupfs in serveral mount points, and then
> umount all of them, kill_sb() will be called only once.
> 
> Therefore it's wrong to increment top_cgroup's refcnt when we find
> an existing cgroup_root.
> 
> Try:
> 	# mount -t cgroup -o cpuacct xxx /cgroup
> 	# mount -t cgroup -o cpuacct xxx /cgroup2
> 	# cat /proc/cgroups | grep cpuacct
> 	cpuacct 2       1       1
> 	# umount /cgroup
> 	# umount /cgroup2
> 	# cat /proc/cgroups | grep cpuacct
> 	cpuacct 2       1       1
> 
> You'll see cgroupfs will never be freed.
> 
> Also move this chunk of code upwards.
> 
> Signed-off-by: Li Zefan <lizefan@...wei.com>
> ---
>  kernel/cgroup.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 37d94a2..5bfe738 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1498,6 +1498,22 @@ retry:
>  		bool name_match = false;
>  
>  		/*
> +		 * A root's lifetime is governed by its top cgroup.  Zero
> +		 * ref indicate that the root is being destroyed.  Wait for
> +		 * destruction to complete so that the subsystems are free.
> +		 * We can use wait_queue for the wait but this path is
> +		 * super cold.  Let's just sleep for a bit and retry.
> +		 */
> +		if (!atomic_read(&root->top_cgroup.refcnt)) {

oops, this fix is wrong. We call kernfs_mount() without cgroup locks and it
drops cgroup refcnt if failed.

I guess we need to bump the refcnt and then drop it after kernfs_mount().
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