netdev - Re: [Patch net] lwt: disable BH too in run_lwt

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Message-ID: <20201201161720.691c58d5@kicinski-fedora-pc1c0hjn.DHCP.thefacebook.com>
Date:   Tue, 1 Dec 2020 16:17:20 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, Dongdong Wang <wangdongdong@...edance.com>,
        Thomas Graf <tgraf@...g.ch>, bpf@...r.kernel.org,
        Cong Wang <cong.wang@...edance.com>
Subject: Re: [Patch net] lwt: disable BH too in run_lwt_bpf()

On Tue, 1 Dec 2020 16:16:33 -0800 Jakub Kicinski wrote:
> On Tue,  1 Dec 2020 11:44:38 -0800 Cong Wang wrote:
> > From: Dongdong Wang <wangdongdong@...edance.com>
> > 
> > The per-cpu bpf_redirect_info is shared among all skb_do_redirect()
> > and BPF redirect helpers. Callers on RX path are all in BH context,
> > disabling preemption is not sufficient to prevent BH interruption.
> > 
> > In production, we observed strange packet drops because of the race
> > condition between LWT xmit and TC ingress, and we verified this issue
> > is fixed after we disable BH.
> > 
> > Although this bug was technically introduced from the beginning, that
> > is commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure"),
> > at that time call_rcu() had to be call_rcu_bh() to match the RCU context.
> > So this patch may not work well before RCU flavor consolidation has been
> > completed around v5.0.
> > 
> > Update the comments above the code too, as call_rcu() is now BH friendly.
> > 
> > Cc: Thomas Graf <tgraf@...g.ch>
> > Cc: bpf@...r.kernel.org
> > Reviewed-by: Cong Wang <cong.wang@...edance.com>
> > Signed-off-by: Dongdong Wang <wangdongdong@...edance.com>  
> 
> Fixes?

Ah, should have read the commit message first. Okay.