Re: [PATCH 04/11] VFS: introduce dentry_lookup_continue()

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Aug 13, 2025 at 1:53 AM NeilBrown <neil@brown.name> wrote:
>
> A few callers operate on a dentry which they already have - unlike the
> normal case where a lookup proceeds an operation.
>
> For these callers dentry_lookup_continue() is provided where other
> callers would use dentry_lookup().  The call will fail if, after the
> lock was gained, the child is no longer a child of the given parent.
>
> There are a couple of callers that want to lock a dentry in whatever
> its current parent is.  For these a NULL parent can be passed, in which
> case ->d_parent is used.  In this case the call cannot fail.
>
> The idea behind the name is that the actual lookup occurred some time
> ago, and now we are continuing with an operation on the dentry.
>
> When the operation completes done_dentry_lookup() must be called.  An
> extra reference is taken when the dentry_lookup_continue() call succeeds
> and will be dropped by done_dentry_lookup().
>
> This will be used in smb/server, ecryptfs, and overlayfs, each of which
> have their own lock_parent() or parent_lock() or similar; and a few
> other places which lock the parent but don't check if the parent is
> still correct (often because rename isn't supported so parent cannot be
> incorrect).
>
> Signed-off-by: NeilBrown <neil@brown.name>
> ---
>  fs/namei.c            | 39 +++++++++++++++++++++++++++++++++++++++
>  include/linux/namei.h |  2 ++
>  2 files changed, 41 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 7af9b464886a..df21b6fa5a0e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1874,6 +1874,45 @@ struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
>  }
>  EXPORT_SYMBOL(dentry_lookup_killable);
>
> +/**
> + * dentry_lookup_continue: lock a dentry if it is still in the given parent, prior to dir ops
> + * @child: the dentry to lock
> + * @parent: the dentry of the assumed parent
> + *
> + * The child is locked - currently by taking i_rwsem on the parent - to
> + * prepare for create/remove operations.  If the given parent is not
> + * %NULL and is no longer the parent of the dentry after the lock is
> + * gained, the lock is released and the call fails (returns
> + * ERR_PTR(-EINVAL).
> + *
> + * On success a reference to the child is taken and returned.  The lock
> + * and reference must both be dropped by done_dentry_lookup() after the
> + * operation completes.
> + */
> +struct dentry *dentry_lookup_continue(struct dentry *child,
> +                                     struct dentry *parent)
> +{
> +       struct dentry *p = parent;
> +
> +again:
> +       if (!parent)
> +               p = dget_parent(child);
> +       inode_lock_nested(d_inode(p), I_MUTEX_PARENT);
> +       if (child->d_parent != p) {

|| d_unhashed(child))

;)

and what about silly renames? are those also d_unhashed()?

Thanks,
Amir.