netdev - Re: [PATCH net-next] dql: annotate data-races around dql->last_obj

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Message-ID: <ZyE4sn-F0ed9YQFQ@LQ3V64L9R2>
Date: Tue, 29 Oct 2024 12:34:10 -0700
From: Joe Damato <jdamato@...tly.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] dql: annotate data-races around
 dql->last_obj_cnt

On Tue, Oct 29, 2024 at 07:14:25PM +0000, Eric Dumazet wrote:
> dql->last_obj_cnt is read/written from different contexts,
> without any lock synchronization.
> 
> Use READ_ONCE()/WRITE_ONCE() to avoid load/store tearing.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/dynamic_queue_limits.h | 2 +-
>  lib/dynamic_queue_limits.c           | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/dynamic_queue_limits.h b/include/linux/dynamic_queue_limits.h
> index 281298e77a1579cba1f92a3b3f03b8be089fd38f..808b1a5102e7c0bbbcd9676b0dacadad2f0ee49a 100644
> --- a/include/linux/dynamic_queue_limits.h
> +++ b/include/linux/dynamic_queue_limits.h
> @@ -127,7 +127,7 @@ static inline void dql_queued(struct dql *dql, unsigned int count)
>  	if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
>  		return;
>  
> -	dql->last_obj_cnt = count;
> +	WRITE_ONCE(dql->last_obj_cnt, count);
>  
>  	/* We want to force a write first, so that cpu do not attempt
>  	 * to get cache line containing last_obj_cnt, num_queued, adj_limit
> diff --git a/lib/dynamic_queue_limits.c b/lib/dynamic_queue_limits.c
> index e49deddd3de9fe9e98d6712559cf48d12a0a2537..c1b7638a594ac43f947e00decabbd3468dcb53de 100644
> --- a/lib/dynamic_queue_limits.c
> +++ b/lib/dynamic_queue_limits.c
> @@ -179,7 +179,7 @@ void dql_completed(struct dql *dql, unsigned int count)
>  
>  	dql->adj_limit = limit + completed;
>  	dql->prev_ovlimit = ovlimit;
> -	dql->prev_last_obj_cnt = dql->last_obj_cnt;
> +	dql->prev_last_obj_cnt = READ_ONCE(dql->last_obj_cnt);
>  	dql->num_completed = completed;
>  	dql->prev_num_queued = num_queued;
>  

This looks fine to me. I noted that dql_reset writes last_obj_cnt,
but AFAIU that write is not a problem (from the 1 driver I looked
at).

Reviewed-by: Joe Damato <jdamato@...tly.com>