Re: [PATCH 03/10] rbtree: Provide rb_find_rcu() / rb_find_add_rcu()

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 08 Jul 2024 11:12:44 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> Much like latch_tree, add two RCU methods for the regular RB-tree,
> which can be used in conjunction with a seqcount to provide lockless
> lookups.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/rbtree.h |   67 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
> 
> --- a/include/linux/rbtree.h
> +++ b/include/linux/rbtree.h
> @@ -245,6 +245,42 @@ rb_find_add(struct rb_node *node, struct
>  }
>  
>  /**
> + * rb_find_add_rcu() - find equivalent @node in @tree, or add @node
> + * @node: node to look-for / insert
> + * @tree: tree to search / modify
> + * @cmp: operator defining the node order
> + *
> + * Adds a Store-Release for link_node.
> + *
> + * Returns the rb_node matching @node, or NULL when no match is found and @node
> + * is inserted.
> + */
> +static __always_inline struct rb_node *
> +rb_find_add_rcu(struct rb_node *node, struct rb_root *tree,
> +		int (*cmp)(struct rb_node *, const struct rb_node *))
> +{
> +	struct rb_node **link = &tree->rb_node;
> +	struct rb_node *parent = NULL;
> +	int c;
> +
> +	while (*link) {

	Don't we need to use rcu_dereference_raw(*link) here?

> +		parent = *link;
> +		c = cmp(node, parent);
> +
> +		if (c < 0)
> +			link = &parent->rb_left;
> +		else if (c > 0)
> +			link = &parent->rb_right;
> +		else
> +			return parent;
> +	}
> +
> +	rb_link_node_rcu(node, parent, link);
> +	rb_insert_color(node, tree);
> +	return NULL;
> +}
> +
> +/**
>   * rb_find() - find @key in tree @tree
>   * @key: key to match
>   * @tree: tree to search
> @@ -268,6 +304,37 @@ rb_find(const void *key, const struct rb
>  		else
>  			return node;
>  	}
> +
> +	return NULL;
> +}
> +
> +/**
> + * rb_find_rcu() - find @key in tree @tree
> + * @key: key to match
> + * @tree: tree to search
> + * @cmp: operator defining the node order
> + *
> + * Notably, tree descent vs concurrent tree rotations is unsound and can result
> + * in false-negatives.
> + *
> + * Returns the rb_node matching @key or NULL.
> + */
> +static __always_inline struct rb_node *
> +rb_find_rcu(const void *key, const struct rb_root *tree,
> +	    int (*cmp)(const void *key, const struct rb_node *))
> +{
> +	struct rb_node *node = tree->rb_node;
> +
> +	while (node) {
> +		int c = cmp(key, node);
> +
> +		if (c < 0)
> +			node = rcu_dereference_raw(node->rb_left);
> +		else if (c > 0)
> +			node = rcu_dereference_raw(node->rb_right);
> +		else
> +			return node;
> +	}
>  
>  	return NULL;
>  }
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>