linux-kernel - Re: [PATCH v2] maple_tree: Add lockdep assertion in mas_dup

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Message-ID: <h54uysfjc62hxcxdnhpj4fnc3smufwiocdkzp3uiah3e2323hc@ljfkxmstxnbo>
Date: Tue, 20 Jan 2026 13:42:06 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Boudewijn van der Heide <boudewijn@...ta-utec.com>
Cc: akpm@...ux-foundation.org, aliceryhl@...gle.com, andrewjballance@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH v2] maple_tree: Add lockdep assertion in mas_dup_alloc()

* Boudewijn van der Heide <boudewijn@...ta-utec.com> [260106 16:08]:
> The __mt_dup() function requires callers to hold the appropriate write
> lock when duplicating a maple tree. Without proper locking, concurrent
> modifications during duplication could access invalid node slots.
> 
> Add a lockdep assertion to catch such API misuse during development.
> This is API hardening rather than a bug fix - all in-tree callers
> already follow the proper locking rules as documented above __mt_dup().
> 
> Signed-off-by: Boudewijn van der Heide <boudewijn@...ta-utec.com>
> ---
> Changes in v2:
>     - Replaced runtime deadnode check with a lockdep assertion
> v1:
>     https://lore.kernel.org/lkml/20260103165758.74094-1-boudewijn@delta-utec.com/
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 5aa4c9500018..3b4357f16352 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -6248,6 +6248,8 @@ static inline void mas_dup_alloc(struct ma_state *mas, struct ma_state *new_mas,
>  	void __rcu **new_slots;
>  	unsigned long val;
>  
> +	lockdep_assert(mt_write_locked(mas->tree));
> +

This is still the wrong place.  You are validating the lock is held in a
function that is called in a loop without any unlocking.

mas_dup_build() is the only caller of mas_dup_alloc(), and that is only
called from two functions: __mt_dup() and mtree_dup().

This would be better served in __mt_dup() since the other caller,
mtree_dup(), already does the locking so there's no way this will
trigger from that call path.

That way, we don't spend a lot of cycles checking lockdep when forking
for no reason.


>  	/* Allocate memory for child nodes. */
>  	type = mte_node_type(mas->node);
>  	new_slots = ma_slots(new_node, type);
> --
> 2.47.3
> 
>