Re: [PATCH 2/2] drivers: core: Remove glue dirs from sysfs earlier

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2018-07-03 at 12:39 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2018-07-02 at 19:26 -0700, Linus Torvalds wrote:
> > On Mon, Jul 2, 2018 at 7:15 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > > 
> > > It's whitespace-damaged on purpose. It's probably broken shit. DO NOT
> > > USE UNDER ANY CIRCUMSTANCES.  Think of it more as a "something like
> > > this might work, but probably doesn't". Maybe it gives you an idea,
> > > although that idea might be "Linus has finally lost it".
> > 
> > Even if it were to work, it should probably just be done inside kernfs
> > and exposed as some kind of "kernfs_remove_if_empty()" function.
> 
> That would be harder, we'd have to expose it via a
> kobject_del_if_empty() then.
> 
> Since we control completely when things are added/removed from the
> gluedir and have a big fat mutex for it, I don't think locking is much
> of an issue. At least in that specific case.

Ok, kernfs was a tad tougher to crack than I hoped but here's a
prototype lightly tested.

Tejun, Greg, does it look reasonable to you ?

Cheers,
Ben.

diff --git a/drivers/base/core.c b/drivers/base/core.c
index b610816eb887..9166f68276c6 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1570,6 +1570,8 @@ static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
 		return;
 
 	mutex_lock(&gdp_mutex);
+	if (!kobject_has_children(glue_dir))
+		kobject_del(glue_dir);
 	kobject_put(glue_dir);
 	mutex_unlock(&gdp_mutex);
 }
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 89d1dc19340b..e4bec09bc602 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1251,6 +1251,29 @@ void kernfs_activate(struct kernfs_node *kn)
 	mutex_unlock(&kernfs_mutex);
 }
 
+bool kernfs_has_children(struct kernfs_node *kn)
+{
+	bool has_children = false;
+	struct kernfs_node *pos;
+
+	/* Lockless shortcut */
+	if (RB_EMPTY_NODE(&kn->rb))
+		return false;
+
+	/* Now check for active children */
+	mutex_lock(&kernfs_mutex);
+	pos = NULL;
+	while ((pos = kernfs_next_descendant_post(pos, kn)) && pos != kn) {
+		if (kernfs_active(pos)) {
+			has_children = true;
+			break;
+		}
+	}
+	mutex_unlock(&kernfs_mutex);
+
+	return has_children;
+}
+
 static void __kernfs_remove(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos;
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index ab25c8b6d9e3..776350ac1cbc 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -300,6 +300,12 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
 	return kn->flags & KERNFS_NS;
 }
 
+/**
+ * kernfs_has_children - Returns whether a kernfs node has children.
+ * @kn: the node to test
+ */
+bool kernfs_has_children(struct kernfs_node *kn);
+
 int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen);
 int kernfs_path_from_node(struct kernfs_node *root_kn, struct kernfs_node *kn,
 			  char *buf, size_t buflen);
diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index 7f6f93c3df9c..1e3294ab95f0 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -116,6 +116,17 @@ extern void kobject_put(struct kobject *kobj);
 extern const void *kobject_namespace(struct kobject *kobj);
 extern char *kobject_get_path(struct kobject *kobj, gfp_t flag);
 
+/**
+ * kobject_has_children - Returns whether a kobject has children.
+ * @kobj: the object to test
+ */
+static inline bool kobject_has_children(struct kobject *kobj)
+{
+	WARN_ON_ONCE(kref_read(&kobj->kref) == 0);
+
+	return kobj->sd && kernfs_has_children(kobj->sd);
+}
+
 struct kobj_type {
 	void (*release)(struct kobject *kobj);
 	const struct sysfs_ops *sysfs_ops;