linux-kernel - Re: [PATCH] netfilter: save the hash of the tuple in the original direction for latter use

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Message-ID: <AANLkTinuUuZSqEzfQAF422x2gYsVLzFQtXZKNLhgw+9g@mail.gmail.com>
Date:	Tue, 17 Aug 2010 16:46:46 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netfilter: save the hash of the tuple in the original
 direction for latter use

On Tue, Aug 17, 2010 at 4:30 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> Three variables ?
>
> static atomic_t rnd __read_mostly;
>
> if (unlikely(!atomic_read(&rnd))) {
>        unsigned int val;
>
>        get_random_bytes(&val, sizeof(val));
>        if (!val)
>                val = 1;
>        atomic_cmpxchg(&rnd, 0, val);
> }
>


Good idea. However, atomic_t is a volatile variable, and it prevent
ILP. I think maybe it hurts the likely case. cmpxchg() is an atomic
operations, so is the bellow code better?

 static unsigned long rnd __read_mostly;

 if (unlikely(!rnd)) {
        unsigned long val;

        get_random_bytes(&val, sizeof(val));
        if (!val)
                val = 1;
        cmpxchg(&rnd, 0, val);
 }

Thanks.

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