linux-kernel - Re: [PATCH 1/2] VFS: Make d_materialise_unique() enforce directory uniqueness

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Date:	Thu, 19 Oct 2006 21:40:56 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Al Viro <viro@....linux.org.uk>, linux-kernel@...r.kernel.org,
	nfs@...ts.sourceforge.net, Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH 1/2] VFS: Make d_materialise_unique() enforce directory
 uniqueness

On Thu, 19 Oct 2006 17:03:58 -0400
Trond Myklebust <Trond.Myklebust@...app.com> wrote:

> +static struct dentry *__d_unalias(struct dentry *dentry, struct dentry *alias)
> +{
> +	struct mutex *m1 = NULL, *m2 = NULL;
> +	struct dentry *ret;
> +
> +	/* If alias and dentry share a parent, then no extra locks required */
> +	if (alias->d_parent == dentry->d_parent)
> +		goto out_unalias;
> +
> +	/* Check for loops */
> +	ret = ERR_PTR(-ELOOP);
> +	if (d_isparent(alias, dentry))
> +		goto out_err;
> +
> +	/* See lock_rename() */
> +	ret = ERR_PTR(-EBUSY);
> +	if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
> +		goto out_err;
> +	m1 = &dentry->d_sb->s_vfs_rename_mutex;
> +	if (!mutex_trylock(&alias->d_parent->d_inode->i_mutex))
> +		goto out_err;
> +	m2 = &alias->d_parent->d_inode->i_mutex;
> +out_unalias:
> +	d_move_locked(alias, dentry);
> +	ret = alias;
> +out_err:
> +	spin_unlock(&dcache_lock);
> +	if (m2)
> +		mutex_unlock(m2);
> +	if (m1)
> +		mutex_unlock(m1);
> +	return ret;
> +}

The locking in there is, of course, gruesome.  There is no way in which it
can be made reliable?

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