[tip: core/rcu] rcu: Add a hlist_nulls_unhashed_lockless() function

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     02b99b38f3d96c77cf0a368d99952aa372dfe58a
Gitweb:        https://git.kernel.org/tip/02b99b38f3d96c77cf0a368d99952aa372dfe58a
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Sat, 09 Nov 2019 10:45:47 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Fri, 10 Jan 2020 14:00:57 -08:00

rcu: Add a hlist_nulls_unhashed_lockless() function

This commit adds an hlist_nulls_unhashed_lockless() to allow lockless
checking for whether or note an hlist_nulls_node is hashed or not.
While in the area, this commit also adds a docbook comment to the existing
hlist_nulls_unhashed() function.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 include/linux/list_nulls.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index 1ecd356..fa6e847 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -56,11 +56,33 @@ static inline unsigned long get_nulls_value(const struct hlist_nulls_node *ptr)
 	return ((unsigned long)ptr) >> 1;
 }
 
+/**
+ * hlist_nulls_unhashed - Has node been removed and reinitialized?
+ * @h: Node to be checked
+ *
+ * Not that not all removal functions will leave a node in unhashed state.
+ * For example, hlist_del_init_rcu() leaves the node in unhashed state,
+ * but hlist_nulls_del() does not.
+ */
 static inline int hlist_nulls_unhashed(const struct hlist_nulls_node *h)
 {
 	return !h->pprev;
 }
 
+/**
+ * hlist_nulls_unhashed_lockless - Has node been removed and reinitialized?
+ * @h: Node to be checked
+ *
+ * Not that not all removal functions will leave a node in unhashed state.
+ * For example, hlist_del_init_rcu() leaves the node in unhashed state,
+ * but hlist_nulls_del() does not.  Unlike hlist_nulls_unhashed(), this
+ * function may be used locklessly.
+ */
+static inline int hlist_nulls_unhashed_lockless(const struct hlist_nulls_node *h)
+{
+	return !READ_ONCE(h->pprev);
+}
+
 static inline int hlist_nulls_empty(const struct hlist_nulls_head *h)
 {
 	return is_a_nulls(READ_ONCE(h->first));