Re: [PATCH v3 21/30] maple_tree: Add cp_is_new_root() helper

[SeongJae Park <sj@kernel.org>]

Hello,

On Fri, 30 Jan 2026 15:59:26 -0500 "Liam R. Howlett" <Liam.Howlett@oracle.com> wrote:

> Add a helper to do what is needed when the maple copy node contains a
> new root node.  This is useful for future commits and is
> self-documenting code.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  lib/maple_tree.c | 70 ++++++++++++++++++++++++++----------------------
>  1 file changed, 38 insertions(+), 32 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 5280fa6d2d6ec..42038e42a4c7e 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3337,6 +3337,43 @@ static void mas_spanning_rebalance(struct ma_state *mas,
>  	mas_spanning_rebalance_loop(mas, mast, count);
>  }
>  
> +static inline bool cp_is_new_root(struct maple_copy *cp, struct ma_state *mas)
> +{
> +	if (cp->min || cp->max != ULONG_MAX)
> +		return false;
> +
> +	if (cp->d_count != 1) {
> +		enum maple_type mt = maple_arange_64;
> +
> +		if (!mt_is_alloc(mas->tree))
> +			mt = maple_range_64;
> +
> +		cp->data = cp->d_count;
> +		cp->s_count = 0;
> +		dst_setup(cp, mas, mt);
> +		init_cp_src(cp);
> +		node_copy(mas, cp->src[0].node, 0, cp->data, cp->max, maple_copy,
> +			  cp->dst[0].node, 0, mt);
> +		node_finalise(cp->dst[0].node, mt, cp->end + 1);
> +		/*
> +		 * Warning, see cp_leaf_init() comment and rcu_assign_pointer()
> +		 * documentation.  Since this is a new root, there are no
> +		 * read-side operations that can view it until it is insert into
> +		 * the tree after an rcu_assign_pointer() call.
> +		 */
> +		RCU_INIT_POINTER(cp->slot[0], mt_mk_node(cp->dst[0].node, mt));

I just found the above makes my build test using an old version compiler fails.
Fortunately, seems it is same to the one we discussed before [1], and same
mitigation like below attached patch works, at least for my test setup.

[1] https://lore.kernel.org/dwhxxuil4zkesmyj6xviyyyfedrcd65h6qd4bplmcrsg36purj@f523i7t6nxag


Thanks,
SJ

[...]
=== >8 ===
From ecc4e468d72c431d53043c8a61fddb6ddf2ecf7c Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Sat, 31 Jan 2026 16:02:56 -0800
Subject: [PATCH] lib/mape_tree: temporal build fix

Without the fix, build with old compilers fails like below:

  CC      lib/maple_tree.o
In file included from .../arch/arm64/include/asm/rwonce.h:67,
                 from .../include/linux/compiler.h:380,
                 from .../include/linux/array_size.h:5,
                 from .../include/linux/kernel.h:16,
                 from .../include/linux/maple_tree.h:11,
                 from .../lib/maple_tree.c:56:
.../lib/maple_tree.c: In function 'cp_is_new_root':
.../include/linux/rcupdate.h:555:36: error: dereferencing pointer to incomplete type 'struct maple_enode'
  555 | #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
      |                                    ^~~~
.../include/asm-generic/rwonce.h:55:33: note: in definition of macro '__WRITE_ONCE'
   55 |  *(volatile typeof(x) *)&(x) = (val);    \
      |                                 ^~~
.../include/linux/rcupdate.h:1046:3: note: in expansion of macro 'WRITE_ONCE'
 1046 |   WRITE_ONCE(p, RCU_INITIALIZER(v)); \
      |   ^~~~~~~~~~
.../include/linux/rcupdate.h:1046:17: note: in expansion of macro 'RCU_INITIALIZER'
 1046 |   WRITE_ONCE(p, RCU_INITIALIZER(v)); \
      |                 ^~~~~~~~~~~~~~~
.../lib/maple_tree.c:3364:3: note: in expansion of macro 'RCU_INIT_POINTER'
 3364 |   RCU_INIT_POINTER(cp->slot[0], mt_mk_node(cp->dst[0].node, mt));
      |   ^~~~~~~~~~~~~~~~

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 lib/maple_tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index c522419e99f4e..eb2855269332a 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3361,7 +3361,8 @@ static inline bool cp_is_new_root(struct maple_copy *cp, struct ma_state *mas)
 		 * read-side operations that can view it until it is insert into
 		 * the tree after an rcu_assign_pointer() call.
 		 */
-		RCU_INIT_POINTER(cp->slot[0], mt_mk_node(cp->dst[0].node, mt));
+		RCU_INIT_POINTER(cp->slot[0],
+				(void *)mt_mk_node(cp->dst[0].node, mt));
 		cp->height++;
 	}
 	WARN_ON_ONCE(cp->dst[0].node != mte_to_node(
-- 
2.47.3