linux-kernel - Re: [PATCH v4] mtd: Do not corrupt backing device of device node inode

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Date:	Fri, 14 May 2010 02:04:34 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Alexander Shishkin <virtuoso@...nd.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	linux-mtd@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
	stable@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mtd: Do not corrupt backing device of device node
 inode

On Wed, 2010-05-05 at 18:40 +0300, Kirill A. Shutemov wrote:
> We cannot modify file->f_mapping->backing_dev_info, because it will corrupt
> backing device of device node inode, since file->f_mapping is equal to
> inode->i_mapping (see __dentry_open() in fs/open.c).
> 
> Let's introduce separate inode for MTD device with appropriate backing
> device.

I hate the fact that we have to do this -- is it really the only option?

Is it _just_ for the backing_device_info? Can't that be done
differently?

> @@ -85,11 +88,27 @@ static int mtd_open(struct inode *inode, struct file *file)
>  		goto out;
>  	}
>  
> -	if (mtd->backing_dev_info)
> -		file->f_mapping->backing_dev_info = mtd->backing_dev_info;
> +	if (!mtd->inode) {
> +		mtd->inode = new_inode(mtd_inode_mnt->mnt_sb);

I believe that would be a race condition, if it wasn't for the BKL.

And what happens when you close the chardevice and call iput() on the
inode so it's destroyed, and then you re-open the device? You never set
mtd->inode = NULL, so won't it now try to igrab a stale pointer?

You won't have seen this in your testing unless you made it prune the
icache between the close and open calls.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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