Re: [PATCH 1/6] rbtree: add rb_find_add_cached() to rbtree.h

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 13, 2024 at 05:51:44PM +1030, Qu Wenruo wrote:
> 
> 
> 在 2024/12/13 03:16, Roger L. Beckermeyer III 写道:
> > Adds rb_find_add_cached() as a helper function for use with
> > red-black trees. Used in btrfs to reduce boilerplate code.
> 
> I won't call it boilerplate code though, it's just to utilize the cached
> rb tree feature as an optimization.

Nah, all this is boilerplate :-)

> > 
> > Suggested-by: Josef Bacik <josef@toxicpanda.com>
> > Signed-off-by: Roger L. Beckermeyer III <beckerlee3@gmail.com>
> > ---
> >   include/linux/rbtree.h | 37 +++++++++++++++++++++++++++++++++++++
> >   1 file changed, 37 insertions(+)
> > 
> > diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
> > index 7c173aa64e1e..0d4444c0cfb3 100644
> > --- a/include/linux/rbtree.h
> > +++ b/include/linux/rbtree.h
> > @@ -210,6 +210,43 @@ rb_add(struct rb_node *node, struct rb_root *tree,
> >   	rb_insert_color(node, tree);
> >   }
> > 
> > +/**
> > + * rb_find_add_cached() - 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
> > + *
> > + * 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_cached(struct rb_node *node, struct rb_root_cached *tree,
> > +	    int (*cmp)(struct rb_node *, const struct rb_node *))
> 
> This function is almost the same as rb_add_cached(), the only difference
> is the extra handling for the cmp function returning 0.
> 
> So I'm wondering if it's possible to enhance rb_add_cached(), or even
> refactor it so there can be a shared core function and rb_add_cached()
> and rb_find_add_cached() can reuse the same function.

Nope, rb_add_cached() can add multiple entries with the same key,
rb_find_add() cannot.

Also, note that all these things are effectively 'templates', they
generate code at the call site. The cmp() function as required for
find_add() is a tri-state return and generates more logic than the
binary less() required for add().