Messages in this thread | | | Date | Wed, 20 Jun 2007 14:26:52 +0530 | From | "Bharata B Rao" <> | Subject | Re: [RFC PATCH 3/4] Lookup changes to support union mount. |
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On 6/20/07, Jan Blunck <jblunck@suse.de> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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