# Re: \[PATCH 1/1\] md/raid10: Remove rcu\_dereference when it doesn't need rcu lock to protect

- From: Song Liu \<song@kernel.org\>
- Date: 2021-08-16 05:23:01 
- List: linux-raid
- Page: https://ratatoskr.run/linux-raid/2021/08/6640565
- Thread: https://ratatoskr.run/linux-raid/2021/08/6640542/t.md

```text
On Fri, Aug 13, 2021 at 6:34 PM Xiao Ni [ref] wrote:
>
> Hi Song
>
> It can improve the performance. It needs to add rcu lock when calling
> rcu_dereference.
> Now it has a bug. It doesn't use rcu lock to protect. In the second
> loop, it doesn't need
> to use rcu_dereference when getting rdev. So to resolve this bug, we can remove
> rcu_dereference directly.

In the second loop, we only use rdev and rrdev when bio and repl_bio
exists. So we shouldn't trigger the "bug" in any cases, right?

Please:
1) If you do think this is a bug, add a fix tag, so we can back port to stable.
   (while I still think it is not a real bug).
2) move struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev); to
  under "if (r10_bio->devs[disk].bio)"; and the rrdev ... to "if
(repl_bio)". And add
  a comment there so it is more clear in the code.

Thanks,
Song
```