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

From: Kirill A. Shutemov
Date: Mon May 17 2010 - 09:19:41 EST

On Fri, May 14, 2010 at 4:04 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> 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?

Yes, it's ugly, but I don't see options.

>> @@ -85,11 +88,27 @@ static int mtd_open(struct inode *inode, struct file *file)
>> Â Â Â Â Â Â Â goto out;
>> Â Â Â }
>> - Â Â if (mtd->backing_dev_info)
>> - Â Â Â Â Â Â file->f_mapping->backing_dev_info = mtd->backing_dev_info;
>> + Â Â if (!mtd->inode) {
>> + Â Â Â Â Â Â mtd->inode = new_inode(mtd_inode_mnt->mnt_sb);
> I believe that would be a race condition, if it wasn't for the BKL.

Ok, I'll fix it.

> 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?

inode destroys only on del_mtd_device() so it's safe to re-open chardevice.

> You won't have seen this in your testing unless you made it prune the
> icache between the close and open calls.
> --
> David Woodhouse              ÂOpen Source Technology Centre
> David.Woodhouse@xxxxxxxxx               ÂIntel Corporation
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