netdev - Re: [PATCH] net: skb content must be visible for lockless skb

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Date:   Mon, 01 Aug 2022 08:52:12 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Kirill Tkhai <tkhai@...ru>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: skb content must be visible for lockless
 skb_peek() and its variations

On Sun, 2022-07-31 at 23:39 +0300, Kirill Tkhai wrote:
> From: Kirill Tkhai <tkhai@...ru>
> 
> Currently, there are no barriers, and skb->xxx update may become invisible on cpu2.
> In the below example var2 may point to intial_val0 instead of expected var1:
> 
> [cpu1]					[cpu2]
> skb->xxx = initial_val0;
> ...
> skb->xxx = var1;			skb = READ_ONCE(prev_skb->next);
> <no barrier>				<no barrier>
> WRITE_ONCE(prev_skb->next, skb);	var2 = skb->xxx;
> 
> This patch adds barriers and fixes the problem. Note, that __skb_peek() is not patched,
> since it's a lowlevel function, and a caller has to understand the things it does (and
> also __skb_peek() is used under queue lock in some places).
> 
> Signed-off-by: Kirill Tkhai <tkhai@...ru>
> ---
> Hi, David, Eric and other developers,
> 
> picking unix sockets code I found this problem, 

Could you please report exactly how/where the problem maifests (e.g.
the involved call paths/time sequence)? 

> and for me it looks like it exists. If there
> are arguments that everything is OK and it's expected, please, explain.

I don't see why such barriers are needed for the locked peek/tail
variants, as the spin_lock pair implies a full memory barrier.

Cheers,

Paolo