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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 12, 2024 at 01:23:43PM -0700, Andrii Nakryiko wrote:
> On Thu, Jul 11, 2024 at 4:07 AM 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>
> > 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?

The store-release when adding it to the tree. Without that it becomes
possible to find the entry while the entry itself is incomplete.

Eg. something like:

 entry.foo = A
 rb_find_add(&entry->node, &my_tree, my_cmp);

vs

 rcu_read_lock();
 entry = rb_find_rcu(...);
 assert(entry->foo == A);

might fail. Because there is nothing ordering the foo store and the
rb-node add.