netdev - RE: [PATCH net] tcp: add a missing barrier in tcp_tasklet

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Date:   Wed, 21 Dec 2016 14:23:53 +0000
From:   Madalin-Cristian Bucur <madalin.bucur@....com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>
CC:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Miffy Lei <xing.lei@....com>
Subject: RE: [PATCH net] tcp: add a missing barrier in tcp_tasklet_func()

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Wednesday, December 21, 2016 3:43 PM
> 
> Madalin reported crashes happening in tcp_tasklet_func() on powerpc64
> 
> Before TSQ_QUEUED bit is cleared, we must ensure the changes done
> by list_del(&tp->tsq_node); are committed to memory, otherwise
> corruption might happen, as an other cpu could catch TSQ_QUEUED
> clearance too soon.
> 
> We can notice that old kernels were immune to this bug, because
> TSQ_QUEUED was cleared after a bh_lock_sock(sk)/bh_unlock_sock(sk)
> section, but they could have missed a kick to write additional bytes,
> when NIC interrupts for a given flow are spread to multiple cpus.
> 
> Affected TCP flows would need an incoming ACK or RTO timer to add more
> packets to the pipe. So overall situation should be better now.
> 
> Fixes: b223feb9de2a ("tcp: tsq: add shortcut in tcp_tasklet_func()")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Madalin Bucur <madalin.bucur@....com>
> Tested-by: Madalin Bucur <madalin.bucur@....com>

It's actually tested by Xing Lei:

Tested-by: Xing Lei <xing.lei@....com>

Thank you for the fast resolution.

> ---
>  net/ipv4/tcp_output.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index
> b45101f3d2bd2e0f0077305a061add4f7ea0de27..31a255b555ad86a3537c077862e3ea38
> f9b44284 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -769,6 +769,7 @@ static void tcp_tasklet_func(unsigned long data)
>  		list_del(&tp->tsq_node);
> 
>  		sk = (struct sock *)tp;
> +		smp_mb__before_atomic();
>  		clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);
> 
>  		if (!sk->sk_lock.owned &&
>