linux-kernel - Re: [PATCH] seqlock: Use while instead of if+goto in __read_seqcount

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Date:   Fri, 10 Apr 2020 12:56:58 +0100
From:   Will Deacon <will@...nel.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     peterz@...radead.org, mingo@...hat.com, mingo@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] seqlock: Use while instead of if+goto in
 __read_seqcount_begin

On Thu, Apr 09, 2020 at 09:45:58PM +0800, Muchun Song wrote:
> The creators of the C language gave us the while keyword. Let's use
> that instead of synthesizing it from if+goto.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  include/linux/seqlock.h | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 8b97204f35a77..7bdea019814ce 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -125,12 +125,8 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
>  {
>  	unsigned ret;
>  
> -repeat:
> -	ret = READ_ONCE(s->sequence);
> -	if (unlikely(ret & 1)) {
> +	while (unlikely((ret = READ_ONCE(s->sequence)) & 1))
>  		cpu_relax();
> -		goto repeat;
> -	}
>  	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
>  	return ret;

Patch looks fine to me, but I'll leave it to Peter as I don't have a
preference either way.

Will