On Fri, Jul 12, 2024 at 01:23:43PM -0700, Andrii Nakryiko wrote: > On Thu, Jul 11, 2024 at 4:07 AM Peter Zijlstra <pet...@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) <pet...@infradead.org> > > Reviewed-by: Masami Hiramatsu (Google) <mhira...@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.
