netdev - Re: [PATCH net-next 03/10] ipv6: np->rxpmtu race annotation

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Message-ID: <27179899-2e67-474e-bb93-82f52c793028@kernel.org>
Date: Wed, 17 Sep 2025 09:34:16 -0600
From: David Ahern <dsahern@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>, "David S . Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org,
 eric.dumazet@...il.com
Subject: Re: [PATCH net-next 03/10] ipv6: np->rxpmtu race annotation

On 9/16/25 10:09 AM, Eric Dumazet wrote:
> Add READ_ONCE() annotations because np->rxpmtu can be changed
> while udpv6_recvmsg() and rawv6_recvmsg() read it.
> 
> Since this is a very rarely used feature, and that udpv6_recvmsg()
> and rawv6_recvmsg() read np->rxopt anyway, change the test order
> so that np->rxpmtu does not need to be in a hot cache line.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv6/raw.c | 2 +-
>  net/ipv6/udp.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c
> index 4ae07a67b4d4f1be6730c252d246e79ff9c73d4c..e369f54844dd9456a819db77435eaef33d162932 100644
> --- a/net/ipv6/raw.c
> +++ b/net/ipv6/raw.c
> @@ -445,7 +445,7 @@ static int rawv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  	if (flags & MSG_ERRQUEUE)
>  		return ipv6_recv_error(sk, msg, len, addr_len);
>  
> -	if (np->rxpmtu && np->rxopt.bits.rxpmtu)
> +	if (np->rxopt.bits.rxpmtu && READ_ONCE(np->rxpmtu))
>  		return ipv6_recv_rxpmtu(sk, msg, len, addr_len);
>  
>  	skb = skb_recv_datagram(sk, flags, &err);
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index b70369f3cd3223cfde07556b1cb1636e8bc78d49..e87d0ef861f88af3ff7bf9dd5045c4d4601036e3 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -479,7 +479,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  	if (flags & MSG_ERRQUEUE)
>  		return ipv6_recv_error(sk, msg, len, addr_len);
>  
> -	if (np->rxpmtu && np->rxopt.bits.rxpmtu)
> +	if (np->rxopt.bits.rxpmtu && READ_ONCE(np->rxpmtu))
>  		return ipv6_recv_rxpmtu(sk, msg, len, addr_len);
>  
>  try_again:

if that pattern is needed again, it should be a helper ...

Reviewed-by: David Ahern <dsahern@...nel.org>