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

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Date:   Mon, 1 Aug 2022 09:39:43 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kirill Tkhai <tkhai@...ru>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        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 Mon, Aug 1, 2022 at 9:00 AM Kirill Tkhai <tkhai@...ru> wrote:
>
> 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?

We prefer explicit _lockless variants to document the precise points
they are needed.

A new helper (and its initial usage) will clearly point to the problem
you saw in af_unix.

BTW, smp_mb__after_spinlock() in your patch does not really make sense to me.
Please add in your changelog the precise issue you are seeing.