Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu_expedited()

[Qiang Zhang <qiang.zhang1211@xxxxxxxxx>]

Qiang Zhang <qiang.zhang1211@xxxxxxxxx> 于2021年10月15日周五 上午10:57写道：

Matthew Wilcox <willy@xxxxxxxxxxxxx> 于2021年10月14日周四 下午7:26写道：

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@xxxxxxxxx>
> Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> ---
>  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?

Thanks for your advice, I find this bdi_tree is traversed under the protection of a spin lock, not under the protection of RCU.

I find this modification does not avoid the problem described in patch, the flush_delayed_work() may be called in release_bdi()

The same will cause problems.

 

may be  we can replace queue_rcu_work() of call_rcu(&inode->i_rcu, i_callback) or do you have any better suggestions?

Thanks

Zqiang