[PATCH] fix klist semantics for lists which have elements removed on traversal

The problem is that klists claim to provide semantics for safe traversal of
lists which are being modified.  The failure case is when traversal of a
list causes element removal (a fairly common case).  The issue is that
although the list node is refcounted, if it is embedded in an object (which
is universally the case), then the object will be freed regardless of the
klist refcount leading to slab corruption because the klist iterator refers
to the prior element to get the next.

The solution is to make the klist take and release references to the
embedding object meaning that the embedding object won't be released until
the list relinquishes the reference to it.

(akpm: fast-track this because it's needed for the 2.6.13 scsi merge)

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/lib/klist.c b/lib/klist.c
index a70c836c5c4c..bb2f3551d50a 100644
--- a/lib/klist.c
+++ b/lib/klist.c
@@ -42,12 +42,23 @@
 /**
  *	klist_init - Initialize a klist structure. 
  *	@k:	The klist we're initializing.
+ *	@get:	The get function for the embedding object (NULL if none)
+ *	@put:	The put function for the embedding object (NULL if none)
+ *
+ * Initialises the klist structure.  If the klist_node structures are
+ * going to be embedded in refcounted objects (necessary for safe
+ * deletion) then the get/put arguments are used to initialise
+ * functions that take and release references on the embedding
+ * objects.
  */
 
-void klist_init(struct klist * k)
+void klist_init(struct klist * k, void (*get)(struct klist_node *),
+		void (*put)(struct klist_node *))
 {
 	INIT_LIST_HEAD(&k->k_list);
 	spin_lock_init(&k->k_lock);
+	k->get = get;
+	k->put = put;
 }
 
 EXPORT_SYMBOL_GPL(klist_init);
@@ -74,6 +85,8 @@ static void klist_node_init(struct klist * k, struct klist_node * n)
 	init_completion(&n->n_removed);
 	kref_init(&n->n_ref);
 	n->n_klist = k;
+	if (k->get)
+		k->get(n);
 }
 
 
@@ -110,9 +123,12 @@ EXPORT_SYMBOL_GPL(klist_add_tail);
 static void klist_release(struct kref * kref)
 {
 	struct klist_node * n = container_of(kref, struct klist_node, n_ref);
+	void (*put)(struct klist_node *) = n->n_klist->put;
 	list_del(&n->n_node);
 	complete(&n->n_removed);
 	n->n_klist = NULL;
+	if (put)
+		put(n);
 }
 
 static int klist_dec_and_del(struct klist_node * n)