netdev - Re: [PATCH net] sit: use ipv6_mod_enabled to check if ipv6 is disabled

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Date:   Tue, 26 Feb 2019 09:43:21 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, Stefano Brivio <sbrivio@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        David Ahern <dsa@...ulusnetworks.com>
Subject: Re: [PATCH net] sit: use ipv6_mod_enabled to check if ipv6 is
 disabled

On Mon, Feb 25, 2019 at 08:39:27AM -0800, Eric Dumazet wrote:
> >> On 02/24/2019 08:12 PM, Hangbin Liu wrote:
> >>> ipv6_mod_enabled() is more safe and gentle to check if ipv6 is disabled
> >>> at running time.
> >>>
> >>
> >>
> >> Why is it better exactly ?
> >>
> >> IPv6 can be enabled on the host, but disabled per device
> >>
> >> /proc/sys/net/ipv6/conf/{name}/disable_ipv6
> > 
> > Sorry, it looks I didn't make it clear in the commit description.
> > This issue only occurs when IPv6 is disabled at boot time as there is
> > no IPv6 route entry. Disable ipv6 on specific interface is not affected.
> > So check ipv6_mod_enabled() is enough and we don't need to worry about
> > the rcu_read_lock or the dev status.
> > 
> > Should I update the commit description?
> 
> Certainly. Are you telling us skb->dev could be NULL here ?
> 
> Because rcu_read_lock() should already be asserted.
> 

No. I know skb->dev is not NULL and we have rcu_read_lock() here. But can we
guarantee the skb->dev won't be NULL forever? Maybe I'm a little sensitive.

I mean for only checking if ipv6 is disable at boot time, use
ipv6_mod_enabled() is more suitable.

Thanks
Hangbin