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

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Date:	Fri, 22 Aug 2008 14:14:58 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"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>,
	Christoph Hellwig <hch@...radead.org>,
	"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, Aug 18, 2008 at 09:28:19PM +0900, Takashi Sato wrote:
> +	down(&bdev->bd_freeze_sem);
> +	bdev->bd_freeze_count++;
> +	if (bdev->bd_freeze_count > 1) {
> +		sb = get_super(bdev);
> +		drop_super(sb);
> +		up(&bdev->bd_freeze_sem);
> +		return sb;
> +	}
> +
>  	down(&bdev->bd_mount_sem);

Now you have a reference counter of freezes which actually is pretty
sensible, but also needs some documentation.  What I don't understand
here at all is why you do the get_super/drop_super in the already frozen
case.

Now that the freeze_count has replaced one of the uses of bd_mount_sem
you should also replace the other use in the unmount path by simply
checking for the freez_count and abort if it's set.  To do so you'll
need to hold the bd_mount_sem over the whole unmount operation to
prevent new frezes from coming in.

As others noted it should be a mutex and not a semaphore.

>  /*
> + * ioctl_freeze - Freeze the filesystem.
> + *
> + * @filp:	target file
> + *
> + * Call freeze_bdev() to freeze the filesystem.
> + */
> +static int ioctl_freeze(struct file *filp)

This is not quite kerneldcoc format, which would ne a /** as commnt
start.  But I don't think the comment is actually needed, it's a pretty
obvious file scope function. (Same commnt also applies to ioctl_thaw)

> +	struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	/* If filesystem doesn't support freeze feature, return. */
> +	if (sb->s_op->write_super_lockfs == NULL)
> +		return -EOPNOTSUPP;
> +
> +	/* If a regular file or a directory isn't specified, return. */
> +	if (sb->s_bdev == NULL)
> +		return -EINVAL;

I don't understand this commnt.  What you are checking is that the
filesystem has a non-NULL s_bdev, which implies a not blockdevice-backed
filesystem.

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