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SubjectRe: 2.5.70-bk16: nfs crash

On Thu, 12 Jun 2003, Linus Torvalds wrote:
>
> If you depend on not re-initializing the pointers, you should not use the
> "xxx_del()" function, and you should document it.

Besides, the code doesn't actually depend on not re-initializing the
pointers, it depends on the _forward_ pointers still being walkable in
case some other CPU is traversing the list just as we remove the entry.

Which means that I think the proper patch is to (a) document this and also
(b) poison the back pointer.

A patch like the attached, in short.

Linus

---
===== include/linux/dcache.h 1.32 vs edited =====
--- 1.32/include/linux/dcache.h Tue Jun 10 14:56:43 2003
+++ edited/include/linux/dcache.h Thu Jun 12 09:12:27 2003
@@ -174,8 +174,10 @@

static inline void __d_drop(struct dentry *dentry)
{
- dentry->d_vfs_flags |= DCACHE_UNHASHED;
- hlist_del_rcu_init(&dentry->d_hash);
+ if (!(dentry->d_vfs_flags & DCACHE_UNHASHED)) {
+ dentry->d_vfs_flags |= DCACHE_UNHASHED;
+ hlist_del_rcu(&dentry->d_hash);
+ }
}

static inline void d_drop(struct dentry *dentry)
===== include/linux/list.h 1.32 vs edited =====
--- 1.32/include/linux/list.h Tue Jun 10 15:46:31 2003
+++ edited/include/linux/list.h Thu Jun 12 08:59:31 2003
@@ -152,14 +152,17 @@
/**
* list_del_rcu - deletes entry from list without re-initialization
* @entry: the element to delete from the list.
+ *
* Note: list_empty on entry does not return true after this,
* the entry is in an undefined state. It is useful for RCU based
* lockfree traversal.
+ *
+ * In particular, it means that we can not poison the forward
+ * pointers that may still be used for path walking.
*/
static inline void list_del_rcu(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
- entry->next = LIST_POISON1;
entry->prev = LIST_POISON2;
}

@@ -431,7 +434,22 @@
n->pprev = LIST_POISON2;
}

-#define hlist_del_rcu hlist_del /* list_del_rcu is identical too? */
+/**
+ * hlist_del_rcu - deletes entry from hash list without re-initialization
+ * @entry: the element to delete from the list.
+ *
+ * Note: list_empty on entry does not return true after this,
+ * the entry is in an undefined state. It is useful for RCU based
+ * lockfree traversal.
+ *
+ * In particular, it means that we can not poison the forward
+ * pointers that may still be used for path walking.
+ */
+static inline void hlist_del_rcu(struct hlist_node *n)
+{
+ __hlist_del(n);
+ n->pprev = LIST_POISON2;
+}

static __inline__ void hlist_del_init(struct hlist_node *n)
{
-
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