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    SubjectRe: [RFC PATCH 3/4] Lookup changes to support union mount.
    Date
    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.

    Jan

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