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Date: Mon, 1 Aug 2022 21:37:05 +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 12:59, Paolo Abeni wrote: > On Mon, 2022-08-01 at 10:00 +0300, Kirill Tkhai 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. > > You mentioned the unix socket, so I expect to see something alike (I'm > totally making up the symbols lists just to give an example): > > CPU0 CPU1 > unix_stream_read_generic() unix_stream_sendmsg() > skb_peek() skb_queue_tail(other->sk_receive_queue) > > plus some wording on how the critical race is reached, if not > completely obvious. Ah, you mean specific functions. Yes, this example and the rest of places, skb_peek{,tail} are used without queue lock, e.g., unix_stream_data_wait(). >> >>>> 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. > > But you are also unconditioanlly adding barriers to the locked > append/enqueue functions - which would possibly make sense only when > the latters are paired with lockless read access. > > As Eric said, if any new barrier is needed, we want to apply it only > where needed, and not to every skb_queue*()/skb_peek() user, so very > likely a new helper (o a new pair of helpers) will be needed. Ok, thanks, Paolo. Kirill
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