Re: [PATCH v2 03/11] rbtree: Provide rb_find_rcu() / rb_find_add_rcu()
From: Andrii Nakryiko <hidden>
Date: 2024-07-12 20:23:56
Also in:
lkml
On Thu, Jul 11, 2024 at 4:07 AM Peter Zijlstra [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.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 *))
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;
+}
+[...]