Re: [PATCH v2 03/11] rbtree: Provide rb_find_rcu() / rb_find_add_rcu()

[Andrii Nakryiko]

On Thu, Jul 11, 2024 at 4:07 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  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 *))

I don't get the point of the RCU version of rb_find_add as RCU itself
doesn't provide enough protection for modification of the tree, right?
So in uprobes code you do rb_find_add_rcu() under uprobes_treelock +
uprobes_seqcount locks. Wouldn't it be just as fine to do plain
non-RCU rb_find_add() in that case? After all, you do plain rb_erase
under the same set of locks.

So what's the point of this one?

> +{
> +       struct rb_node **link = &tree->rb_node;
> +       struct rb_node *parent = NULL;
> +       int c;
> +
> +       while (*link) {
> +               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;
> +}
> +

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