linux-kernel - Re: [PATCH v2] raid5-cache: use try_cmpxchg in r5l_wake

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Date:   Thu, 20 Oct 2022 11:09:25 -0700
From:   Song Liu <song@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] raid5-cache: use try_cmpxchg in r5l_wake_reclaim

On Thu, Oct 20, 2022 at 8:51 AM Uros Bizjak <ubizjak@...il.com> wrote:
>
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> r5l_wake_reclaim. 86 CMPXCHG instruction returns success in ZF flag, so
> this change saves a compare after cmpxchg (and related move instruction in
> front of cmpxchg).
>
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> fails. There is no need to re-read the value in the loop.
>
> Note that the value from *ptr should be read using READ_ONCE to prevent
> the compiler from merging, refetching or reordering the read.
>
> No functional change intended.
>
> Cc: Song Liu <song@...nel.org>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

Applied to md-next. Thanks!

Song

> ---
> v2: Fix garbled subject line
> ---
>  drivers/md/raid5-cache.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 832d8566e165..a63023aae21e 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -1565,11 +1565,12 @@ void r5l_wake_reclaim(struct r5l_log *log, sector_t space)
>
>         if (!log)
>                 return;
> +
> +       target = READ_ONCE(log->reclaim_target);
>         do {
> -               target = log->reclaim_target;
>                 if (new < target)
>                         return;
> -       } while (cmpxchg(&log->reclaim_target, target, new) != target);
> +       } while (!try_cmpxchg(&log->reclaim_target, &target, new));
>         md_wakeup_thread(log->reclaim_thread);
>  }
>
> --
> 2.37.3
>