netdev - Re: [PATCH net-next-2.6] net: build_ehash_secret() and rt_bind

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Date:	Fri, 20 Aug 2010 00:10:55 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH net-next-2.6] net: build_ehash_secret() and rt_bind_peer() cleanups

On Thu, Aug 19, 2010 at 11:58 PM, Changli Gao <xiaosuo@...il.com> wrote:
> On Thu, Aug 19, 2010 at 11:46 PM, Eric Dumazet <eric.dumazet@...il.com> 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 <eric.dumazet@...il.com>
>> CC: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
>> ---
>>  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);
>>  }
>>  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);
>>
>> -       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))
>
> It isn't correct, and should be
>     if (unlikely(cmpxchg(&rt->peer, NULL, peer) != NULL))
>

we should also test peer != NULL.


-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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