linux-kernel - Re: [PATCH RESEND 2/2] ucount: Use atomic_long_try_cmpxchg in atomic_long_inc

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Date:   Mon, 31 Oct 2022 20:02:12 +0100
From:   Alexey Gladkov <legion@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH RESEND 2/2] ucount: Use atomic_long_try_cmpxchg in
 atomic_long_inc_below

On Mon, Oct 17, 2022 at 04:40:49PM +0200, Uros Bizjak wrote:
> Use atomic_long_try_cmpxchg instead of
> atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_below.
> x86 CMPXCHG instruction returns success in ZF flag, so this change saves
> a compare after cmpxchg (and related move instruction in front of cmpxchg).
> 
> Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value to "old"
> when cmpxchg fails, enabling further code simplifications.
> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>

Reviewed-by: Alexey Gladkov <legion@...nel.org>

Eric, I think the patch looks good. You already reviewed the previous
version [1]. What do you think about this version?

[1] https://lore.kernel.org/lkml/87pmi6z143.fsf@email.froward.int.ebiederm.org/

> ---
>  kernel/ucount.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 74f5b4959feb..2c04589a61ff 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -214,16 +214,14 @@ void put_ucounts(struct ucounts *ucounts)
>  
>  static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
>  {
> -	long c, old;
> -	c = atomic_long_read(v);
> -	for (;;) {
> +	long c = atomic_long_read(v);
> +
> +	do {
>  		if (unlikely(c >= u))
>  			return false;
> -		old = atomic_long_cmpxchg(v, c, c+1);
> -		if (likely(old == c))
> -			return true;
> -		c = old;
> -	}
> +	} while (!atomic_long_try_cmpxchg(v, &c, c+1));
> +
> +	return true;
>  }
>  
>  struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
> -- 
> 2.37.3
> 

-- 
Rgrds, legion