linux-kernel - Re: [PATCH 1/2] vfs: Provide function to get superblock and wait for it to thaw

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Date:	Thu, 9 Feb 2012 10:20:49 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, mpatocka@...hat.com,
	sandeen@...deen.net
Subject: Re: [PATCH 1/2] vfs: Provide function to get superblock and wait for
 it to thaw

On Tue, Feb 07, 2012 at 11:37:46PM +0100, Jan Kara wrote:
> In quota code we need to find a superblock corresponding to a device and wait
> for superblock to be unfrozen. However this waiting has to happen without
> s_umount semaphore because that is required for superblock to thaw. So provide
> a function in VFS for this to keep dances with s_umount where they belong.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/super.c         |   47 +++++++++++++++++++++++++++++++++++------------
>  include/linux/fs.h |    1 +
>  2 files changed, 36 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 6015c02..e320239 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -593,15 +593,7 @@ void iterate_supers_type(struct file_system_type *type,
>  
>  EXPORT_SYMBOL(iterate_supers_type);
>  
> -/**
> - *	get_super - get the superblock of a device
> - *	@bdev: device to get the superblock for
> - *	
> - *	Scans the superblock list and finds the superblock of the file system
> - *	mounted on the device given. %NULL is returned if no match is found.
> - */
> -
> -struct super_block *get_super(struct block_device *bdev)
> +static struct super_block *__get_super(struct block_device *bdev, bool thawed)
>  {
>  	struct super_block *sb;
>  
> @@ -618,9 +610,13 @@ rescan:
>  			spin_unlock(&sb_lock);
>  			down_read(&sb->s_umount);
>  			/* still alive? */
> -			if (sb->s_root && (sb->s_flags & MS_BORN))
> -				return sb;
> -			up_read(&sb->s_umount);
> +			if (sb->s_root && (sb->s_flags & MS_BORN)) {
> +				if (sb->s_frozen == SB_UNFROZEN)
> +					return sb;
> +				up_read(&sb->s_umount);
> +				vfs_check_frozen(sb, SB_FREEZE_WRITE);
> +			} else
> +				up_read(&sb->s_umount);
>  			/* nope, got unmounted */
>  			spin_lock(&sb_lock);
>  			__put_super(sb);
> @@ -631,9 +627,36 @@ rescan:
>  	return NULL;
>  }

The thawed parameter appears to be unused and the waiting for frozen
filesystems appears to happen for all callers. Is this intentional?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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