linux-ext4 - Re: [PATCH 1/3] Implement generic freeze feature

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Date:	Mon, 8 Sep 2008 13:10:26 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"mtk.manpages@...glemail.com" <mtk.manpages@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Implement generic freeze feature

On Mon, Sep 08, 2008 at 08:52:45PM +0900, Takashi Sato wrote:
> diff -uprN -X linux-2.6.27-rc5.org/Documentation/dontdiff linux-2.6.27-rc5.org/fs/block_dev.c linux-2.6.27-rc5-freeze/fs
> /block_dev.c
> --- linux-2.6.27-rc5.org/fs/block_dev.c	2008-08-29 07:52:02.000000000 +0900
> +++ linux-2.6.27-rc5-freeze/fs/block_dev.c	2008-09-05 20:00:29.000000000 +0900
> @@ -285,6 +285,8 @@ static void init_once(void *foo)
>  	INIT_LIST_HEAD(&bdev->bd_holder_list);
>  #endif
>  	inode_init_once(&ei->vfs_inode);
> +	/* Initialize mutex for freeze. */
> +	mutex_init(&bdev->bd_fsfreeze_mutex);

Why not just freeze_mutex?


>  struct super_block *freeze_bdev(struct block_device *bdev)
>  {
>  	struct super_block *sb;
>  
> +	mutex_lock(&bdev->bd_fsfreeze_mutex);
> +	if (bdev->bd_fsfreeze_count > 0) {
> +		bdev->bd_fsfreeze_count++;
> +		sb = get_super(bdev);
> +		mutex_unlock(&bdev->bd_fsfreeze_mutex);
> +		return sb;
> +	}
> +	bdev->bd_fsfreeze_count++;
> +
>  	down(&bdev->bd_mount_sem);

Note that we still have duplication with the bd_mount_sem.  I think
you should look into getting rid of it and instead do a check of
the freeze_count under proper freeze_mutex protection.

> +int thaw_bdev(struct block_device *bdev, struct super_block *sb)
>  {
> +	mutex_lock(&bdev->bd_fsfreeze_mutex);
> +	if (!bdev->bd_fsfreeze_count) {
> +		mutex_unlock(&bdev->bd_fsfreeze_mutex);
> +		return 0;
> +	}
> +
> +	bdev->bd_fsfreeze_count--;
> +	if (bdev->bd_fsfreeze_count > 0) {
> +		if (sb)
> +			drop_super(sb);
> +		mutex_unlock(&bdev->bd_fsfreeze_mutex);
> +		return 0;
> +	}
> +
>  	if (sb) {
>  		BUG_ON(sb->s_bdev != bdev);
>  
> @@ -244,6 +274,8 @@ void thaw_bdev(struct block_device *bdev
>  	}
>  
>  	up(&bdev->bd_mount_sem);
> +	mutex_unlock(&bdev->bd_fsfreeze_mutex);
> +	return 0;

Why do you add a return value here if we always return 0 anyway?

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