linux-ext4 - Re: [PATCH 02/12] container quota: lock/unlock mount namespace when performing quotactl.

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Date:	Thu, 31 May 2012 13:04:24 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	<jeff.liu@...cle.com>
CC:	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<jack@...e.cz>, <daniel.lezcano@...e.fr>, <tytso@....edu>,
	<bpm@....com>, <chris.mason@...cle.com>, <hch@...radead.org>,
	<christopher.jones@...cle.com>, <david@...morbit.com>,
	<tinguely@....com>, <tm@....ma>, <linux-ext4@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 02/12] container quota: lock/unlock mount namespace when
 performing quotactl.

On 05/30/2012 06:58 PM, jeff.liu@...cle.com wrote:
> Introduce two functions to lock/unlock a particular mount namespace
> when container quota operations is performed.
>
> Signed-off-by: Jie Liu<jeff.liu@...cle.com>
> ---
>   fs/namespace.c                |   17 +++++++++++++++++
>   include/linux/mnt_namespace.h |   13 +++++++++++++
>   2 files changed, 30 insertions(+), 0 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index e608199..8823ba8 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2633,3 +2633,20 @@ bool our_mnt(struct vfsmount *mnt)
>   {
>   	return check_mnt(real_mount(mnt));
>   }
> +
> +/*
> + * Lock the mount namespace when performing a particular container
> + * disk quotactl operation, especially to protect removing it.
> + */
> +void lock_mnt_ns(struct mnt_namespace *ns)
> +{
> +	down_write(&namespace_sem);
> +}
> +
> +/*
> + * Unlock the mount namespace after quotactl on that was done.
> + */
> +void unlock_mnt_ns(struct mnt_namespace *ns)
> +{
> +	up_write(&namespace_sem);
> +}
> diff --git a/include/linux/mnt_namespace.h b/include/linux/mnt_namespace.h
> index 5a8e390..ff72ed5 100644
> --- a/include/linux/mnt_namespace.h
> +++ b/include/linux/mnt_namespace.h
> @@ -9,6 +9,19 @@ extern struct mnt_namespace *copy_mnt_ns(unsigned long, struct mnt_namespace *,
>   		struct fs_struct *);
>   extern void put_mnt_ns(struct mnt_namespace *ns);
>
> +#ifdef CONFIG_NS_QUOTA
> +extern void lock_mnt_ns(struct mnt_namespace *);
> +extern void unlock_mnt_ns(struct mnt_namespace *);
> +#else
> +void lock_mnt_ns(struct mnt_namespace *)
> +{
> +}
> +
> +void unlock_mnt_ns(struct mnt_namespace *)
> +{
> +}
> +#endif
> +

Seems weird. There is nothing ns-quota specific in locking a namespace.
Also, just grabbing a lock from an outer subsystem is quite ugly. If you 
are grabbing a namespace lock, this suggests you are tweaking namespace 
data. So you should have that file provide you with a function that does 
the full operation.

Of course there are exceptions to that, but it rings a bell.
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