Subject: Re: [PATCH net-next-2.6] net: build_ehash_secret() and rt_bind_peer() cleanups

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 19 Aug 2010 12:28:05 -0400

* Eric Dumazet ([email-masked]) wrote:
> Now cmpxchg() is available on all arches, we can use it in
> build_ehash_secret() and rt_bind_peer() instead of using spinlocks.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> CC: Mathieu Desnoyers <[email-masked]>
> ---
>  net/ipv4/af_inet.c |    8 +++-----
>  net/ipv4/route.c   |   11 +++--------
>  2 files changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index 6a1100c..f581f77 100644
> --- a/net/ipv4/af_inet.c
> +++ b/net/ipv4/af_inet.c
> @@ -227,18 +227,16 @@ EXPORT_SYMBOL(inet_ehash_secret);
>  
>  /*
>   * inet_ehash_secret must be set exactly once
> - * Instead of using a dedicated spinlock, we (ab)use inetsw_lock
>   */
>  void build_ehash_secret(void)
>  {
>  	u32 rnd;
> +
>  	do {
>  		get_random_bytes(&rnd, sizeof(rnd));
>  	} while (rnd == 0);
> -	spin_lock_bh(&inetsw_lock);
> -	if (!inet_ehash_secret)
> -		inet_ehash_secret = rnd;
> -	spin_unlock_bh(&inetsw_lock);
> +
> +	cmpxchg(&inet_ehash_secret, 0, rnd);

I'd be more comfortable if you add a comment saying why you can get away
with not testing the cmpxchg failure case. (it's because the failure is
caused by a concurrent CPU setting the ehash secret to a random value,
which actually does the job for us, so no need to retry).

>  }
>  EXPORT_SYMBOL(build_ehash_secret);
>  
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 3f56b6e..ae3dba7 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1268,18 +1268,13 @@ skip_hashing:
>  
>  void rt_bind_peer(struct rtable *rt, int create)
>  {
> -	static DEFINE_SPINLOCK(rt_peer_lock);
>  	struct inet_peer *peer;
>  
>  	peer = inet_getpeer(rt->rt_dst, create);

Hrm, peer can be NULL on OOM condition.

>  
> -	spin_lock_bh(&rt_peer_lock);
> -	if (rt->peer == NULL) {
> -		rt->peer = peer;
> -		peer = NULL;
> -	}
> -	spin_unlock_bh(&rt_peer_lock);
> -	if (peer)
> +	peer = cmpxchg(&rt->peer, NULL, peer);
> +
> +	if (unlikely(peer))
>  		inet_putpeer(peer);

And you don't want to put the peer returned by cmpxchg, you want to put
the peer you just allocated.

I'd go for:

  if (!peer)
    return;
  if (unlikely(cmpxchg(&rt->peer, NULL, peer) != NULL))
    inet_putpeer(peer);

Thanks,

Mathieu

>  }

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com