[PATCH 6/8] rbtree: Implement generic latch_tree

[Peter Zijlstra]

Implement a latched RB-tree in order to get unconditional RCU/lockless
lookups.

Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Woodhouse <David.Woodhouse@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/rbtree_latch.h |  223 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 223 insertions(+)

--- /dev/null
+++ b/include/linux/rbtree_latch.h
@@ -0,0 +1,223 @@
+/*
+ * Latched RB-trees
+ *
+ * Copyright (C) 2015 Intel Corp., Peter Zijlstra <peterz@xxxxxxxxxxxxx>
+ *
+ * Since RB-trees have non atomic modifications they're not immediately suited
+ * for RCU/lockless queries. Even though we made RB tree lookups non-fatal for
+ * lockless lookups; we cannot guarantee they return a correct result.
+ *
+ * The simplest solution is a seqlock + rb-tree, this will allow lockless
+ * lookups; but has the constraint (inherent to the seqlock) that read sides
+ * cannot nest in write sides.
+ *
+ * If we need to allow unconditional lookups (say as required for NMI context
+ * usage) we need a more complex setup; this data structure provides this by
+ * employing the latch technique -- see @raw_write_seqcount_latch -- to
+ * implement a latched RB-tree which does allow for unconditional lookups by
+ * virtue of always having (at least) one stable copy of the tree.
+ *
+ * However, while we have the guarantee that there is at all times one stable
+ * copy, this does not guarantee an iteration will not observe modifications.
+ * What might have been a stable copy at the start of the iteration, need not
+ * remain so for the duration of the iteration.
+ *
+ * Therefore, this does require a lockless RB-tree iteration to be non-fatal;
+ * see the comment in lib/rbtree.c. Note however that we only require the first
+ * condition -- not seeing partial stores -- because the latch thing isolates
+ * us from loops. If we were to interrupt a modification the lookup would be
+ * pointed at the stable tree and complete while the modification was halted.
+ */
+
+#ifndef RB_TREE_LATCH_H
+#define RB_TREE_LATCH_H
+
+#include <linux/rbtree.h>
+#include <linux/seqlock.h>
+
+struct latch_tree_node {
+	/*
+	 * Because we have an array of two entries in struct latch_tree_nodes
+	 * its not possible to use container_of() to get back to the
+	 * encapsulating structure; therefore we have to put in a back pointer.
+	 */
+	void		*priv;
+	struct rb_node	node;
+};
+
+struct latch_tree_nodes {
+	struct latch_tree_node node[2];
+};
+
+struct latch_tree_root {
+	seqcount_t	seq;
+	struct rb_root	tree[2];
+};
+
+/**
+ * latch_tree_ops - operators to define the tree order
+ * @less: used for insertion; provides the (partial) order between two elements.
+ * @comp: used for lookups; provides the order between the search key and an element.
+ *
+ * The operators are related like:
+ *
+ *	comp(a->key,b) < 0  := less(a,b)
+ *	comp(a->key,b) > 0  := less(b,a)
+ *	comp(a->key,b) == 0 := !less(a,b) && !less(b,a)
+ *
+ * If these operators define a partial order on the elements we make no
+ * guarantee on which of the elements matching the key is found. See
+ * latch_tree_find().
+ */
+struct latch_tree_ops {
+	bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b);
+	int  (*comp)(void *key,                 struct latch_tree_node *b);
+};
+
+static __always_inline void
+__lt_insert(struct latch_tree_node *ltn, struct rb_root *root,
+	    bool (*less)(struct latch_tree_node *a, struct latch_tree_node *b))
+{
+	struct rb_node **link = &root->rb_node;
+	struct rb_node *parent = NULL;
+	struct latch_tree_node *ltp;
+
+	while (*link) {
+		parent = *link;
+		ltp = container_of(parent, struct latch_tree_node, node);
+
+		if (less(ltn, ltp))
+			link = &parent->rb_left;
+		else
+			link = &parent->rb_right;
+	}
+
+	rb_link_node_rcu(&ltn->node, parent, link);
+	rb_insert_color(&ltn->node, root);
+}
+
+static __always_inline void
+__lt_erase(struct latch_tree_node *ltn, struct rb_root *root)
+{
+	rb_erase(&ltn->node, root);
+}
+
+static __always_inline struct latch_tree_node *
+__lt_find(void *key, struct rb_root *root,
+	  int (*comp)(void *key, struct latch_tree_node *ltn))
+{
+	struct rb_node *n = rcu_dereference_raw(root->rb_node);
+	struct latch_tree_node *ltn;
+	int c;
+
+	while (n) {
+		ltn = container_of(n, struct latch_tree_node, node);
+		c = comp(key, ltn);
+
+		if (c < 0)
+			n = rcu_dereference_raw(n->rb_left);
+		else if (c > 0)
+			n = rcu_dereference_raw(n->rb_right);
+		else
+			return ltn;
+	}
+
+	return NULL;
+}
+
+/**
+ * latch_tree_insert() - insert @nodes into the trees @root
+ * @nodes: nodes to insert
+ * @root: trees to insert @nodes into
+ * @priv: pointer to node private data
+ * @ops: operators defining the node order
+ *
+ * Initializes @nodes private pointer with @priv; which typically is a back
+ * pointer to the containing structure, used by @ops to find the search key.
+ *
+ * Then it inserts @nodes into @root in an ordered fashion such that we can
+ * always observe one complete tree. See the comment for
+ * raw_write_seqcount_latch().
+ *
+ * The inserts use rcu_assign_pointer() to publish the element such that
+ * both the @priv pointer values in @nodes as the tree structure is stored
+ * before we can observe the new @nodes.
+ *
+ * All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
+ * serialized.
+ */
+static __always_inline void
+latch_tree_insert(struct latch_tree_nodes *nodes,
+		  struct latch_tree_root *root,
+		  void *priv,
+		  const struct latch_tree_ops *ops)
+{
+	nodes->node[0].priv = nodes->node[1].priv = priv;
+
+	raw_write_seqcount_latch(&root->seq);
+	__lt_insert(&nodes->node[0], &root->tree[0], ops->less);
+	raw_write_seqcount_latch(&root->seq);
+	__lt_insert(&nodes->node[1], &root->tree[1], ops->less);
+}
+
+/**
+ * latch_tree_erase() - removes @nodes from the trees @root
+ * @nodes: nodes to remote
+ * @root: trees to remove @nodes from
+ * @ops: operators defining the node order
+ *
+ * Removes @nodes from the trees @root in an ordered fashion such that we can
+ * always observe one complete tree. See the comment for
+ * raw_write_seqcount_latch().
+ *
+ * It is assumed that @nodes will observe one RCU quiesent state before being
+ * reused of freed.
+ *
+ * All modifications (latch_tree_insert, latch_tree_remove) are assumed to be
+ * serialized.
+ */
+static __always_inline void
+latch_tree_erase(struct latch_tree_nodes *nodes,
+		 struct latch_tree_root *root,
+		 const struct latch_tree_ops *ops)
+{
+	raw_write_seqcount_latch(&root->seq);
+	__lt_erase(&nodes->node[0], &root->tree[0]);
+	raw_write_seqcount_latch(&root->seq);
+	__lt_erase(&nodes->node[1], &root->tree[1]);
+}
+
+/**
+ * latch_tree_find() - find the node matching @key in the trees @root
+ * @key: search key
+ * @root: trees to search for @key
+ * @ops: operators defining the node order
+ *
+ * Does a lockless lookup in the trees @root for the node matching @key.
+ *
+ * It is assumed that this is called while holding the appropriate RCU read
+ * side lock.
+ *
+ * If the operators define a partial order on the elements (there are multiple
+ * elements which have the same key value) it is undefined which of these
+ * elements will be found. Nor is it possible to iterate the tree to find
+ * further elements with the same key value.
+ *
+ * Returns: a pointer to the node matching @key or NULL.
+ */
+static __always_inline struct latch_tree_node *
+latch_tree_find(void *key, struct latch_tree_root *root,
+		const struct latch_tree_ops *ops)
+{
+	struct latch_tree_node *node;
+	unsigned int seq;
+
+	do {
+		seq = raw_read_seqcount(&root->seq);
+		node = __lt_find(key, &root->tree[seq & 1], ops->comp);
+	} while (read_seqcount_retry(&root->seq, seq));
+
+	return node;
+}
+
+#endif /* RB_TREE_LATCH_H */


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/