netdev - Re: [PATCH net] ipv6: fix suspecious RCU usage warning

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Date:   Mon, 8 Mar 2021 19:47:31 -0700
From:   David Ahern <dsahern@...il.com>
To:     Wei Wang <weiwan@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Ido Schimmel <idosch@...dia.com>,
        Petr Machata <petrm@...dia.com>
Cc:     syzbot <syzkaller@...glegroups.com>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] ipv6: fix suspecious RCU usage warning

[ cc Ido and Petr ]

On 3/8/21 12:21 PM, Wei Wang wrote:
> diff --git a/include/net/nexthop.h b/include/net/nexthop.h
> index 7bc057aee40b..48956b144689 100644
> --- a/include/net/nexthop.h
> +++ b/include/net/nexthop.h
> @@ -410,31 +410,39 @@ static inline struct fib_nh *fib_info_nh(struct fib_info *fi, int nhsel)
>  int fib6_check_nexthop(struct nexthop *nh, struct fib6_config *cfg,
>  		       struct netlink_ext_ack *extack);
>  
> -static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
> +static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh,
> +					      bool bh_disabled)

Hi Wei: I would prefer not to have a second argument to nexthop_fib6_nh
for 1 code path, and a control path at that.

>  {
>  	struct nh_info *nhi;
>  
>  	if (nh->is_group) {
>  		struct nh_group *nh_grp;
>  
> -		nh_grp = rcu_dereference_rtnl(nh->nh_grp);
> +		if (bh_disabled)
> +			nh_grp = rcu_dereference_bh_rtnl(nh->nh_grp);
> +		else
> +			nh_grp = rcu_dereference_rtnl(nh->nh_grp);
>  		nh = nexthop_mpath_select(nh_grp, 0);
>  		if (!nh)
>  			return NULL;
>  	}
>  
> -	nhi = rcu_dereference_rtnl(nh->nh_info);
> +	if (bh_disabled)
> +		nhi = rcu_dereference_bh_rtnl(nh->nh_info);
> +	else
> +		nhi = rcu_dereference_rtnl(nh->nh_info);
>  	if (nhi->family == AF_INET6)
>  		return &nhi->fib6_nh;
>  
>  	return NULL;
>  }
>  

I am wary of duplicating code, but this helper is simple enough that it
should be ok with proper documentation.

Ido/Petr: I think your resilient hashing patch set touches this helper.
How ugly does it get to have a second version?