From: john cooper <john.cooper@redhat.com>
Date: Sat, 4 Apr 2009 16:28:12 -0400
Subject: [misc] introduce list_del_init_rcu
Message-id: 49D7C2DC.6050008@redhat.com
O-Subject: [RHEL5.4 PATCH] implement mmu_notifier mechanism V3, [1/5] BZ#485718
Bugzilla: 485718
RH-Acked-by: Rik van Riel <riel@redhat.com>
RH-Acked-by: Rik van Riel <riel@redhat.com>

BZ#485718  https://bugzilla.redhat.com/show_bug.cgi?id=485718

Introduce list_del_init_rcu() and document it.

diff --git a/include/linux/list.h b/include/linux/list.h
index 7ff5183..0107208 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -781,6 +781,34 @@ static inline void hlist_del_init(struct hlist_node *n)
 	}
 }
 
+/**
+ * hlist_del_init_rcu - deletes entry from hash list with re-initialization
+ * @n: the element to delete from the hash list.
+ *
+ * Note: list_unhashed() on the node return true after this. It is
+ * useful for RCU based read lockfree traversal if the writer side
+ * must know if the list entry is still hashed or already unhashed.
+ *
+ * In particular, it means that we can not poison the forward pointers
+ * that may still be used for walking the hash list and we can only
+ * zero the pprev pointer so list_unhashed() will return true after
+ * this.
+ *
+ * The caller must take whatever precautions are necessary (such as
+ * holding appropriate locks) to avoid racing with another
+ * list-mutation primitive, such as hlist_add_head_rcu() or
+ * hlist_del_rcu(), running on this same list.  However, it is
+ * perfectly legal to run concurrently with the _rcu list-traversal
+ * primitives, such as hlist_for_each_entry_rcu().
+ */
+static inline void hlist_del_init_rcu(struct hlist_node *n)
+{
+	if (!hlist_unhashed(n)) {
+		__hlist_del(n);
+		n->pprev = NULL;
+	}
+}
+
 /*
  * hlist_replace_rcu - replace old entry by new one
  * @old : the element to be replaced