On Wed, 2014-01-15 at 10:17 -0500, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
>
> d_splice_alias can create duplicate directory aliases (in the !new
> case), or (in the new case) d_move without holding appropriate locks.
It can d_move, because the dentry is known to be disconnected, i.e. it
doesn't have a parent for which we could obtain the lock.
> d_materialise_unique deals with both of these problems. (The latter
> seems to be dealt by trylocks (see __d_unalias), which look like they
> could cause spurious lookup failures--but that's at least better than
> corrupting the dcache.)
>
> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> ---
> fs/dcache.c | 25 +------------------------
> 1 file changed, 1 insertion(+), 24 deletions(-)
>
> Only lightly tested.... If this is right, then we can also just ditch
> d_splice_alias completely, and clean up the various d_find_alias's.
>
> I think the only reason we have both d_splice_alias and
> d_materialise_unique is that the former was written for exportable
> filesystems and the latter for distributed filesystems.
>
> But we have at least one exportable filesystem (fuse) using
> d_materialise_unique. And I doubt d_splice_alias was ever completely
> correct even for on-disk filesystems.
>
> Am I missing some subtlety?
One subtle difference is that for a non-directory d_splice_alias() will
reconnect a DCACHE_DISCONNECTED dentry if one exists, while
d_materialise_unique() will not.
Does this matter in practice? The small number of extra dentries
probably does not matter.
Thanks,
Miklos
>
> --b.
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 4bdb300..da82fa9 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1926,33 +1926,10 @@ EXPORT_SYMBOL(d_obtain_alias);
> */
> struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
> {
> - struct dentry *new = NULL;
> -
> if (IS_ERR(inode))
> return ERR_CAST(inode);
>
> - if (inode && S_ISDIR(inode->i_mode)) {
> - spin_lock(&inode->i_lock);
> - new = __d_find_alias(inode, 1);
> - if (new) {
> - BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
> - spin_unlock(&inode->i_lock);
> - security_d_instantiate(new, inode);
> - d_move(new, dentry);
> - iput(inode);
> - } else {
> - /* already taking inode->i_lock, so d_add() by hand */
> - __d_instantiate(dentry, inode);
> - spin_unlock(&inode->i_lock);
> - security_d_instantiate(dentry, inode);
> - d_rehash(dentry);
> - }
> - } else {
> - d_instantiate(dentry, inode);
> - if (d_unhashed(dentry))
> - d_rehash(dentry);
> - }
> - return new;
> + return d_materialise_unique(dentry, inode);
> }
> EXPORT_SYMBOL(d_splice_alias);
>
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