Date: Thu, 3 Feb 2005 16:49:22 -0800
From: "David S. Miller" <davem@davemloft.net>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: anton@samba.org, okir@suse.de, netdev@oss.sgi.com,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arp_queue: serializing unlink + kfree_skb
Message-Id: <20050203164922.2627a112.davem@davemloft.net>
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On Fri, 4 Feb 2005 10:50:44 +1100
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> So the problem isn't as big as I thought which is good.  sk_buff
> is only in trouble because of the atomic_read optimisation which
> really needs a memory barrier.
> 
> However, instead of adding a memory barrier which makes the optimisation
> less useful, let's just get rid of the atomic_read.

See my other email, the atomic_read() should function just fine.

If we see the count dropped to "1", whoever set it to "1" made
sure that all outstanding memory operations (including things
like __skb_unlink()) are globally visible before the
atomic_dec_and_test() which put the thing to "1" from "2".
(and we did use atomic_dec_and_test() since the refcount was
 not "1")  Example, assuming skb->users is "2":

	cpu 0			cpu 1
				__skb_unlink()
				kfree_skb()
	kfree_skb()

If cpu 0 sees the count at "1", it will always see the
__skb_unlink() as well.

Either my logic is flawed (very possible, I am a pinhead) or something
is amiss in the PPC atomic ops.

I describe all of this more explicitly in my other email.
I'm actually going through all the sparc64 chip manuals to make
sure I have things correct in that implementation :-)))
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