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

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Date:   Mon, 1 Aug 2022 10:00:33 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Paolo Abeni <pabeni@...hat.com>,
        "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 01.08.2022 09:52, Paolo Abeni wrote:
> 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)?

I didn't get why call paths in the patch description are not enough for you. Please, explain
what you want.
 
>> 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.

This is for lockless skb_peek() calls and the patch is called in that way :). For locked skb_peek()
this is not needed. I'm not sure we need separate skb_peek() and skb_peek_lockless(). Do we?