linux-kernel - Re: [PATCH 03/10] rbtree: Provide rb_find_rcu() / rb_find_add

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Message-Id: <20240710230521.db73990a46ca2f927dd0e8a5@kernel.org>
Date: Wed, 10 Jul 2024 23:05:21 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, andrii@...nel.org, linux-kernel@...r.kernel.org,
 rostedt@...dmis.org, oleg@...hat.com, jolsa@...nel.org, clm@...a.com,
 paulmck@...nel.org
Subject: Re: [PATCH 03/10] rbtree: Provide rb_find_rcu() / rb_find_add_rcu()

On Wed, 10 Jul 2024 09:55:57 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Jul 10, 2024 at 10:29:59AM +0900, Masami Hiramatsu wrote:
> > On Mon, 08 Jul 2024 11:12:44 +0200
> > Peter Zijlstra <peterz@...radead.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@...radead.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 *))
> > > +{
> > > +	struct rb_node **link = &tree->rb_node;
> > > +	struct rb_node *parent = NULL;
> > > +	int c;
> > > +
> > > +	while (*link) {
> > 
> > 	Don't we need to use rcu_dereference_raw(*link) here?
> 
> This is a modifying operation and as such we can assume operation under
> the exclusive lock. IOW the tree should be stable here.

Ah, got it.

> 
> > > +		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);
> 
> Only the link operation needs the rcu_assign_pointer() thing for
> publishing our new node.

Yes.

> 
> > > +	rb_insert_color(node, tree);
> 
> The rotations use WRITE_ONCE() to avoid tearing.

OK, thanks for confirmation.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> 
> > > +	return NULL;
> > > +}


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>