[tip/core/rcu,6/9] rcu: Add a hlist_nulls_unhashed_lockless() function
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Message ID 20191210042002.3490-6-paulmck@kernel.org
State In Next
Commit 50a3d82d959a1a663291aa6f4b5f125038c460df
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Series
  • RCU list updates for v5.6
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Commit Message

Paul E. McKenney Dec. 10, 2019, 4:19 a.m. UTC
From: "Paul E. McKenney" <paulmck@kernel.org>

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@kernel.org>
---
 include/linux/list_nulls.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

Patch
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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));