linux-kernel - Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu

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Date:   Thu, 14 Oct 2021 12:24:22 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zqiang <qiang.zhang1211@...il.com>
Cc:     akpm@...ux-foundation.org, sunhao.th@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of
 synchronize_rcu_expedited()

On Thu, Oct 14, 2021 at 04:24:33PM +0800, Zqiang wrote:
> The bdi_remove_from_list() is called in RCU softirq, however the
> synchronize_rcu_expedited() will produce sleep action, use kfree_rcu()
> instead of it.
> 
> Reported-by: Hao Sun <sunhao.th@...il.com>
> Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
> ---
>  include/linux/backing-dev-defs.h | 1 +
>  mm/backing-dev.c                 | 4 +---
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 33207004cfde..35a093384518 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -202,6 +202,7 @@ struct backing_dev_info {
>  #ifdef CONFIG_DEBUG_FS
>  	struct dentry *debug_dir;
>  #endif
> +	struct rcu_head rcu;
>  };

Instead of growing struct backing_dev_info, it seems to me this rcu_head
could be placed in a union with rb_node, since it will have been removed
from the bdi_tree by this point and the tree is never walked under
RCU protection?