netdev - Re: [PATCH net-next] ipv4: igmp: annotate data-races around idev->mr

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Message-ID: <60f5b633-d214-450c-a1ce-26df8f2d8f26@kernel.org>
Date: Thu, 22 Jan 2026 13:05:34 -0700
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>, netdev@...r.kernel.org,
 eric.dumazet@...il.com
Subject: Re: [PATCH net-next] ipv4: igmp: annotate data-races around
 idev->mr_maxdelay

On 1/22/26 10:22 AM, Eric Dumazet wrote:
> idev->mr_maxdelay is read and written locklessly,
> add READ_ONCE()/WRITE_ONCE() annotations.
> 
> While we are at it, make this field an u32.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/inetdevice.h | 2 +-
>  net/ipv4/igmp.c            | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/inetdevice.h b/include/linux/inetdevice.h
> index 5730ba6b1cfaff25f2b9d206722662e1e62b88fc..dccbeb25f70141982160c776f3cc727296def2b7 100644
> --- a/include/linux/inetdevice.h
> +++ b/include/linux/inetdevice.h
> @@ -38,11 +38,11 @@ struct in_device {
>  	struct ip_mc_list	*mc_tomb;
>  	unsigned long		mr_v1_seen;
>  	unsigned long		mr_v2_seen;
> -	unsigned long		mr_maxdelay;
>  	unsigned long		mr_qi;		/* Query Interval */
>  	unsigned long		mr_qri;		/* Query Response Interval */
>  	unsigned char		mr_qrv;		/* Query Robustness Variable */
>  	unsigned char		mr_gq_running;
> +	u32			mr_maxdelay;
>  	u32			mr_ifc_count;
>  	struct timer_list	mr_gq_timer;	/* general query timer */
>  	struct timer_list	mr_ifc_timer;	/* interface change timer */
> diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
> index 7182f1419c2a4d4ea1e90ef6857f6af895dd2770..0adc993c211d7a3901f9a0e9ee816e7fddc598cd 100644
> --- a/net/ipv4/igmp.c
> +++ b/net/ipv4/igmp.c
> @@ -227,7 +227,7 @@ static void igmp_start_timer(struct ip_mc_list *im, int max_delay)
>  
>  static void igmp_gq_start_timer(struct in_device *in_dev)
>  {
> -	int tv = get_random_u32_below(in_dev->mr_maxdelay);
> +	int tv = get_random_u32_below(READ_ONCE(in_dev->mr_maxdelay));

u32 tv? fix that type while at it.

>  	unsigned long exp = jiffies + tv + 2;
>  
>  	if (in_dev->mr_gq_running &&
> @@ -1009,7 +1009,7 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
>  		max_delay = IGMPV3_MRC(ih3->code)*(HZ/IGMP_TIMER_SCALE);
>  		if (!max_delay)
>  			max_delay = 1;	/* can't mod w/ 0 */
> -		in_dev->mr_maxdelay = max_delay;
> +		WRITE_ONCE(in_dev->mr_maxdelay, max_delay);

max_delay can also be u32; only scope creep with this patch set is
fixing up igmp_mod_timer to take u32.


>  
>  		/* RFC3376, 4.1.6. QRV and 4.1.7. QQIC, when the most recently
>  		 * received value was zero, use the default or statically

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