klist: don't iterate over deleted entries

A klist entry is kept on the list till all its current iterations are
finished; however, a new iteration after deletion also iterates over
deleted entries as long as their reference count stays above zero.
This causes problems for cases where there are users which iterate
over the list while synchronized against list manipulations and
natuarally expect already deleted entries to not show up during
iteration.

This patch implements dead flag which gets set on deletion so that
iteration can skip already deleted entries. The dead flag piggy backs
on the lowest bit of knode->n_klist and only visible to klist
implementation proper.

While at it, drop klist_iter->i_head as it's redundant and doesn't
offer anything in semantics or performance wise as klist_iter->i_klist
is dereferenced on every iteration anyway.

Signed-off-by: Tejun Heo <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Alan Stern <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/klist.h

···
 		       void (*put)(struct klist_node *));
 
 struct klist_node {
-	struct klist		*n_klist;
+	void			*n_klist;	/* never access directly */
 	struct list_head	n_node;
 	struct kref		n_ref;
 	struct completion	n_removed;
···
 
 struct klist_iter {
 	struct klist		*i_klist;
-	struct list_head	*i_head;
 	struct klist_node	*i_cur;
 };
 

lib/klist.c

···
 #include <linux/klist.h>
 #include <linux/module.h>
 
+/*
+ * Use the lowest bit of n_klist to mark deleted nodes and exclude
+ * dead ones from iteration.
+ */
+#define KNODE_DEAD		1LU
+#define KNODE_KLIST_MASK	~KNODE_DEAD
+
+static struct klist *knode_klist(struct klist_node *knode)
+{
+	return (struct klist *)
+		((unsigned long)knode->n_klist & KNODE_KLIST_MASK);
+}
+
+static bool knode_dead(struct klist_node *knode)
+{
+	return (unsigned long)knode->n_klist & KNODE_DEAD;
+}
+
+static void knode_set_klist(struct klist_node *knode, struct klist *klist)
+{
+	knode->n_klist = klist;
+	/* no knode deserves to start its life dead */
+	WARN_ON(knode_dead(knode));
+}
+
+static void knode_kill(struct klist_node *knode)
+{
+	/* and no knode should die twice ever either, see we're very humane */
+	WARN_ON(knode_dead(knode));
+	*(unsigned long *)&knode->n_klist |= KNODE_DEAD;
+}
 
 /**
  * klist_init - Initialize a klist structure.
···
 	INIT_LIST_HEAD(&n->n_node);
 	init_completion(&n->n_removed);
 	kref_init(&n->n_ref);
-	n->n_klist = k;
+	knode_set_klist(n, k);
 	if (k->get)
 		k->get(n);
 }
···
  */
 void klist_add_after(struct klist_node *n, struct klist_node *pos)
 {
-	struct klist *k = pos->n_klist;
+	struct klist *k = knode_klist(pos);
 
 	klist_node_init(k, n);
 	spin_lock(&k->k_lock);
···
  */
 void klist_add_before(struct klist_node *n, struct klist_node *pos)
 {
-	struct klist *k = pos->n_klist;
+	struct klist *k = knode_klist(pos);
 
 	klist_node_init(k, n);
 	spin_lock(&k->k_lock);
···
 {
 	struct klist_node *n = container_of(kref, struct klist_node, n_ref);
 
+	WARN_ON(!knode_dead(n));
 	list_del(&n->n_node);
 	complete(&n->n_removed);
-	n->n_klist = NULL;
+	knode_set_klist(n, NULL);
 }
 
 static int klist_dec_and_del(struct klist_node *n)
 {
 	return kref_put(&n->n_ref, klist_release);
+}
+
+static void klist_put(struct klist_node *n, bool kill)
+{
+	struct klist *k = knode_klist(n);
+	void (*put)(struct klist_node *) = k->put;
+
+	spin_lock(&k->k_lock);
+	if (kill)
+		knode_kill(n);
+	if (!klist_dec_and_del(n))
+		put = NULL;
+	spin_unlock(&k->k_lock);
+	if (put)
+		put(n);
 }
 
 /**
···
  */
 void klist_del(struct klist_node *n)
 {
-	struct klist *k = n->n_klist;
-	void (*put)(struct klist_node *) = k->put;
-
-	spin_lock(&k->k_lock);
-	if (!klist_dec_and_del(n))
-		put = NULL;
-	spin_unlock(&k->k_lock);
-	if (put)
-		put(n);
+	klist_put(n, true);
 }
 EXPORT_SYMBOL_GPL(klist_del);
 
···
 			  struct klist_node *n)
 {
 	i->i_klist = k;
-	i->i_head = &k->k_list;
 	i->i_cur = n;
 	if (n)
 		kref_get(&n->n_ref);
···
 void klist_iter_exit(struct klist_iter *i)
 {
 	if (i->i_cur) {
-		klist_del(i->i_cur);
+		klist_put(i->i_cur, false);
 		i->i_cur = NULL;
 	}
 }
···
  */
 struct klist_node *klist_next(struct klist_iter *i)
 {
-	struct list_head *next;
-	struct klist_node *lnode = i->i_cur;
-	struct klist_node *knode = NULL;
 	void (*put)(struct klist_node *) = i->i_klist->put;
+	struct klist_node *last = i->i_cur;
+	struct klist_node *next;
 
 	spin_lock(&i->i_klist->k_lock);
-	if (lnode) {
-		next = lnode->n_node.next;
-		if (!klist_dec_and_del(lnode))
+
+	if (last) {
+		next = to_klist_node(last->n_node.next);
+		if (!klist_dec_and_del(last))
 			put = NULL;
 	} else
-		next = i->i_head->next;
+		next = to_klist_node(i->i_klist->k_list.next);
 
-	if (next != i->i_head) {
-		knode = to_klist_node(next);
-		kref_get(&knode->n_ref);
+	i->i_cur = NULL;
+	while (next != to_klist_node(&i->i_klist->k_list)) {
+		if (likely(!knode_dead(next))) {
+			kref_get(&next->n_ref);
+			i->i_cur = next;
+			break;
+		}
+		next = to_klist_node(next->n_node.next);
 	}
-	i->i_cur = knode;
+
 	spin_unlock(&i->i_klist->k_lock);
-	if (put && lnode)
-		put(lnode);
-	return knode;
+
+	if (put && last)
+		put(last);
+	return i->i_cur;
 }
 EXPORT_SYMBOL_GPL(klist_next);