On 6/20/07, Jan Blunck <jblunck@xxxxxxx> wrote:
On Wed, 20 Jun 2007 11:23:26 +0530, Bharata B Rao wrote:
> +/*
> + * Looks for the given @name in dcache by walking through all the layers
> + * of the union stack, starting from the top.
> + * FIXME: If we don't find the dentry in a upper layer, we descend to the
> + * next layer. So there is a chance to miss this dentry in the top layer
> + * if this is the _first_ time lookup of the dentry in this layer. A real
> + * lookup might have fetched a valid dentry in this layer itself, while we
> + * chose to descend to the next lower layer. One solution is not have this
> + * function itself, do the toplevel lookup in dcache and if it fails proceed
> + * to real_lookup_union() directly.
> + */
> +struct dentry *__d_lookup_union(struct nameidata *nd, struct qstr *name)
> +{
> + struct dentry *dentry;
> + struct nameidata nd_tmp;
> + struct vfsmount *mnt = mntget(nd->mnt);
> + struct qstr this;
> + int err;
> +
> + nd_tmp.mnt = nd->mnt;
> + nd_tmp.dentry = nd->dentry;
> +
> + this.name = name->name;
> + this.len = name->len;
> + this.hash = name->hash;
> +
> + do {
> + /* d_hash() is a repetition for the top layer. */
> + if (nd_tmp.dentry->d_op && nd_tmp.dentry->d_op->d_hash) {
> + err = nd_tmp.dentry->d_op->d_hash(nd_tmp.dentry, &this);
> + if (err < 0)
> + goto out;
> + }
> +
> + dentry = __d_lookup(nd_tmp.dentry, &this);
> + if (dentry) {
> + if (dentry->d_inode) {
> + if (nd->mnt != nd_tmp.mnt) {
> + mntput(nd->mnt);
> + nd->mnt = mntget(nd_tmp.mnt);
> + }
> + mntput(mnt);
> + return dentry;
> + } else {
> + dput(dentry);
> + }
> + }
> + } while (next_union_mount(&nd_tmp));
> +out:
> + mntput(mnt);
> + return NULL;
> +}
> +
The reference counting for vfsmount is wrong. next_union_mount() should be
something similar to follow_down(). You should grab the reference to the
underlying mount before doing the lookup. Ok ok, you already have a valid
reference in struct union_mount but anyway.
The idea is that a reference to the top layer's vfsmount and dentry is
taken by the user of the union mount point/directory (eg. lookup,
readdir etc) and that should ensure that all the bottom layers are
valid until the top level reference is held.
Regards,
Bharata.
--
"Men come and go but mountains remain" -- Ruskin Bond.
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