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

- From: Xiao Ni \<hidden\>
- Date: 2021-08-14 01:34:23 
- List: linux-raid
- Page: https://ratatoskr.run/linux-raid/2021/08/6640564
- Thread: https://ratatoskr.run/linux-raid/2021/08/6640542/t.md

```text
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.

Best Regards
Xiao

On Sat, Aug 14, 2021 at 12:50 AM Song Liu [ref] wrote:
>
> On Sun, Aug 8, 2021 at 9:02 PM Xiao Ni [ref] wrote:
> >
> > In the first loop of function raid10_handle_discard. It already
> > determines which disk need to handle discard request and add the
> > rdev reference count. So the conf->mirrors will not change until
> > all bios come back from underlayer disks. It doesn't need to use
> > rcu_dereference to get rdev.
> >
> > Signed-off-by: Xiao Ni <redacted>
>
> Will we get performance benefits from this change? If not, I would
> prefer to keep the code as-is.
>
> Thanks,
> Song
>
> > ---
> >  drivers/md/raid10.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> > index 16977e8..cef9869 100644
> > --- a/drivers/md/raid10.c
> > +++ b/drivers/md/raid10.c
> > @@ -1743,9 +1743,8 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
> >         for (disk = 0; disk < geo->raid_disks; disk++) {
> >                 sector_t dev_start, dev_end;
> >                 struct bio *mbio, *rbio = NULL;
> > -               struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
> > -               struct md_rdev *rrdev = rcu_dereference(
> > -                       conf->mirrors[disk].replacement);
> > +               struct md_rdev *rdev = conf->mirrors[disk].rdev;
> > +               struct md_rdev *rrdev = conf->mirrors[disk].replacement;
> >
> >                 /*
> >                  * Now start to calculate the start and end address for each disk.
> > --
> > 2.7.5
> >
>

```