hlist_head is often used for implementing bucket header of
hash table, and one lock is often needed for adding/deleting
node in the bucket list. It can consume lots of memory if
per-bucket spinlock is used, so this patch trys to use the 1st
bit of hlist_head->first as bit lock for this purpose.
bit spinlock isn't efficient as spin lock, but the contention
shouldn't be very high for operating per-bucket hlist, so bit
spinlock should be OK.
Signed-off-by: Ming Lei <tom.leim...@gmail.com>
---
include/linux/rculist.h | 55 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 14ec165..9fc394a 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -435,6 +435,61 @@ static inline void hlist_replace_rcu(struct hlist_node
*old,
#define hlist_next_rcu(node) (*((struct hlist_node __rcu **)(&(node)->next)))
#define hlist_pprev_rcu(node) (*((struct hlist_node __rcu **)((node)->pprev)))
+/*
+ * This bit of hlist_head->first can be used as per-bucket
+ * bit spin_lock in hash table use case, and the lock bit
+ * should always be kept when hlist_node is added, deleted
+ * and other update operations.
+ *
+ * When lock bit is used, only the _lock version of hlist
+ * helpers can be used for operating the hlist.
+ */
+#define HLIST_LOCK_BIT 0
+#define HLIST_LOCK_MASK (1UL << HLIST_LOCK_BIT)
+#define hlist_get_first(h) \
+ (struct hlist_node *)((unsigned long)(h)->first & ~HLIST_LOCK_MASK)
+#define hlist_get_lock(h) ((unsigned long)(h)->first & HLIST_LOCK_MASK)
+#define hlist_make_1st_node(h, n) \
+ (struct hlist_node *)((unsigned long)(n) | hlist_get_lock(h))
+
+static inline struct hlist_head *hlist_get_head_lock(
+ struct hlist_head *head, struct hlist_head *new_head)
+{
+ new_head->first = hlist_get_first(head);
+ return new_head;
+}
+
+static inline void __hlist_del_lock(struct hlist_node *n)
+{
+ struct hlist_node *next = n->next;
+ struct hlist_node **pprev = n->pprev;
+
+ WRITE_ONCE(*pprev, (unsigned long)next |
+ ((unsigned long)*pprev & HLIST_LOCK_MASK));
+ if (next)
+ next->pprev = pprev;
+}
+
+/* The bit lock should be held before calling this helper */
+static inline void hlist_del_rcu_lock(struct hlist_node *n)
+{
+ __hlist_del_lock(n);
+ n->pprev = LIST_POISON2;
+}
+
+/* The bit lock should be held before calling this helper */
+static inline void hlist_add_head_rcu_lock(struct hlist_node *n,
+ struct hlist_head *h)
+{
+ struct hlist_node *first = hlist_get_first(h);
+
+ n->next = first;
+ n->pprev = &h->first;
+ rcu_assign_pointer(hlist_first_rcu(h), hlist_make_1st_node(h, n));
+ if (first)
+ first->pprev = &n->next;
+}
+
/**
* hlist_add_head_rcu
* @n: the element to add to the hash list.
--
1.9.1
--
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