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

[Andrii Nakryiko]

On Mon, Jul 15, 2024 at 4:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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@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?
>
> 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.
>
>

Ah, I see, thanks for the explanation. That's what "Adds a
Store-Release for link_node." in the comment means, I see.