netdev - Re: [PATCH net-next 43/43] netfilter: Skip unnecessary calls to synchronize

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Message-ID: <20150617172054.GJ13215@acer.localdomain>
Date:	Wed, 17 Jun 2015 19:20:54 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	Stephen Hemminger <stephen@...workplumber.org>,
	Juanjo Ciarlante <jjciarla@...z.uncu.edu.ar>,
	Wensong Zhang <wensong@...ux-vs.org>,
	Simon Horman <horms@...ge.net.au>,
	Julian Anastasov <ja@....bg>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH net-next 43/43] netfilter: Skip unnecessary calls to
 synchronize_net

On 17.06, Eric W. Biederman wrote:
> From: Eric W Biederman <ebiederm@...ssion.com>
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  net/netfilter/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/core.c b/net/netfilter/core.c
> index 95456c09cf69..1b4eadc9c030 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -134,7 +134,9 @@ void nf_unregister_hook(struct net *net, const struct nf_hook_ops *reg)
>  #ifdef HAVE_JUMP_LABEL
>  	static_key_slow_dec(&nf_hooks_needed[reg->pf][reg->hooknum]);
>  #endif
> -	synchronize_net();
> +	/* Don't wait if there are no packets in flight */
> +	if (net->loopback_dev)
> +		synchronize_net();

I don't get this, could you please explain why there wouldn't be any packets
in flight if there is no loopback_dev?

>  	kfree(elem);
>  }
>  EXPORT_SYMBOL(nf_unregister_hook);
> -- 
> 2.2.1
> 
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