Currently there is no primitive for retrieving the previous list member.
To do this we need a new deletion primitive that doesn't poison the prev
pointer and a corresponding retrieval helper. Note that it is not valid
to ues both list_del_rcu() and list_bidir_del_rcu() on the same list.
Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
include/linux/rculist.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 14dfa6008467e803d57f98cfa0275569f1c6a181..270a9ee2f7976b1736545667973265a3bfb7ec41 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -30,6 +30,17 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
* way, we must not access it directly
*/
#define list_next_rcu(list) (*((struct list_head __rcu **)(&(list)->next)))
+/*
+ * Return the ->prev pointer of a list_head in an rcu safe way. Don't
+ * access it directly.
+ *
+ * Any list traversed with list_bidir_prev_rcu() must never use
+ * list_del_rcu(). Doing so will poison the ->prev pointer that
+ * list_bidir_prev_rcu() relies on, which will result in segfaults.
+ * To prevent these segfaults, use list_bidir_del_rcu() instead
+ * of list_del_rcu().
+ */
+#define list_bidir_prev_rcu(list) (*((struct list_head __rcu **)(&(list)->prev)))
/**
* list_tail_rcu - returns the prev pointer of the head of the list
@@ -158,6 +169,42 @@ static inline void list_del_rcu(struct list_head *entry)
entry->prev = LIST_POISON2;
}
+/**
+ * list_bidir_del_rcu - deletes entry from list without re-initialization
+ * @entry: the element to delete from the list.
+ *
+ * In contrast to list_del_rcu() doesn't poison the prev pointer thus
+ * allowing backwards traversal via list_bidir_prev_rcu().
+ *
+ * Note: list_empty() on entry does not return true after this because
+ * the entry is in a special undefined state that permits RCU-based
+ * lockfree reverse traversal. In particular this means that we can not
+ * poison the forward and backwards pointers that may still be used for
+ * walking the list.
+ *
+ * The caller must take whatever precautions are necessary (such as
+ * holding appropriate locks) to avoid racing with another list-mutation
+ * primitive, such as list_bidir_del_rcu() or list_add_rcu(), running on
+ * this same list. However, it is perfectly legal to run concurrently
+ * with the _rcu list-traversal primitives, such as
+ * list_for_each_entry_rcu().
+ *
+ * Noe that the it is not allowed to use list_del_rcu() and
+ * list_bidir_del_rcu() on the same list.
+ *
+ * Note that list_del_rcu() and list_bidir_del_rcu() must not be used on
+ * the same list.
+ *
+ * Note that the caller is not permitted to immediately free
+ * the newly deleted entry. Instead, either synchronize_rcu()
+ * or call_rcu() must be used to defer freeing until an RCU
+ * grace period has elapsed.
+ */
+static inline void list_bidir_del_rcu(struct list_head *entry)
+{
+ __list_del_entry(entry);
+}
+
/**
* hlist_del_init_rcu - deletes entry from hash list with re-initialization
* @n: the element to delete from the hash list.
--
2.45.2
