netdev - Re: [PATCH net 2/2] rfs: annotate lockless accesses to RFS sock flow table

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Message-ID: <20230602202753.51130-1-kuniyu@amazon.com>
Date: Fri, 2 Jun 2023 13:27:53 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <eric.dumazet@...il.com>, <kuba@...nel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>, <kuniyu@...zon.com>
Subject: Re: [PATCH net 2/2] rfs: annotate lockless accesses to RFS sock flow table

From: Eric Dumazet <edumazet@...gle.com>
Date: Fri,  2 Jun 2023 16:31:41 +0000
> Add READ_ONCE()/WRITE_ONCE() on accesses to the sock flow table.
> 
> This also prevents a (smart ?) compiler to remove the condition in:
> 
> if (table->ents[index] != newval)
>         table->ents[index] = newval;
> 
> We need the condition to avoid dirtying a shared cache line.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Thanks!


> ---
>  include/linux/netdevice.h | 7 +++++--
>  net/core/dev.c            | 6 ++++--
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 08fbd4622ccf731daaee34ad99773d6dc2e82fa6..e6f22b7403d014a2cf4d81d931109a594ce1398e 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -768,8 +768,11 @@ static inline void rps_record_sock_flow(struct rps_sock_flow_table *table,
>  		/* We only give a hint, preemption can change CPU under us */
>  		val |= raw_smp_processor_id();
>  
> -		if (table->ents[index] != val)
> -			table->ents[index] = val;
> +		/* The following WRITE_ONCE() is paired with the READ_ONCE()
> +		 * here, and another one in get_rps_cpu().
> +		 */
> +		if (READ_ONCE(table->ents[index]) != val)
> +			WRITE_ONCE(table->ents[index], val);
>  	}
>  }
>  
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b3c13e0419356b943e90b1f46dd7e035c6ec1a9c..1495f8aff288e944c8cab21297f244a6fcde752f 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4471,8 +4471,10 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
>  		u32 next_cpu;
>  		u32 ident;
>  
> -		/* First check into global flow table if there is a match */
> -		ident = sock_flow_table->ents[hash & sock_flow_table->mask];
> +		/* First check into global flow table if there is a match.
> +		 * This READ_ONCE() pairs with WRITE_ONCE() from rps_record_sock_flow().
> +		 */
> +		ident = READ_ONCE(sock_flow_table->ents[hash & sock_flow_table->mask]);
>  		if ((ident ^ hash) & ~rps_cpu_mask)
>  			goto try_rps;
>  
> -- 
> 2.41.0.rc0.172.g3f132b7071-goog