linux-kernel - Re: [PATCH] maple_tree: Remove rcu_read_lock() from mt

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Message-Id: <20240820175417.2782532-1-Liam.Howlett@oracle.com>
Date: Wed, 21 Aug 2024 06:38:45 +0800
From: Hillf Danton <hdanton@...a.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	maple-tree@...ts.infradead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	syzbot+036af2f0c7338a33b0cd@...kaller.appspotmail.com
Subject: Re: [PATCH] maple_tree: Remove rcu_read_lock() from mt_validate()

On Tue, 20 Aug 2024 13:54:17 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> The write lock should be held when validating the tree to avoid updates
> racing with checks.  Holding the rcu read lock during a large tree
> validation may also cause a prolonged rcu read window.
> 
>From the rcu stall's view, holding spin lock plays the same role of rcu
read lock, so what are you fixing by simply dropping rcu read lock?

> Link: https://lore.kernel.org/all/0000000000001d12d4062005aea1@google.com/
> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Reported-by: syzbot+036af2f0c7338a33b0cd@...kaller.appspotmail.com
> ---
>  lib/maple_tree.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 755ba8b18e14..fe1b01b29201 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -7588,14 +7588,14 @@ static void mt_validate_nulls(struct maple_tree *mt)
>   * 2. The gap is correctly set in the parents
>   */
>  void mt_validate(struct maple_tree *mt)
> +	__must_hold(mas->tree->ma_lock)
>  {
>  	unsigned char end;
>  
	lockdep_assert_held(ma_lock); instead

>  	MA_STATE(mas, mt, 0, 0);
> -	rcu_read_lock();
>  	mas_start(&mas);
>  	if (!mas_is_active(&mas))
> -		goto done;
> +		return;
>  
>  	while (!mte_is_leaf(mas.node))
>  		mas_descend(&mas);
> @@ -7616,9 +7616,6 @@ void mt_validate(struct maple_tree *mt)
>  		mas_dfs_postorder(&mas, ULONG_MAX);
>  	}
>  	mt_validate_nulls(mt);
> -done:
> -	rcu_read_unlock();
> -
>  }
>  EXPORT_SYMBOL_GPL(mt_validate);
>  
> -- 
> 2.43.0