netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Date:   Tue, 29 Jan 2019 09:59:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, jakub.kicinski@...ronome.com,
        netdev@...r.kernel.org, kernel-team@...com, mingo@...hat.com,
        will.deacon@....com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        jannh@...gle.com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Mon, Jan 28, 2019 at 01:37:12PM -0800, Alexei Starovoitov wrote:
> On Mon, Jan 28, 2019 at 09:43:10AM +0100, Peter Zijlstra wrote:

> > Isn't that still broken? AFAIU networking progs can happen in task
> > context (TX) and SoftIRQ context (RX), which can nest.
> 
> Sure. sendmsg side of networking can be interrupted by napi receive.
> Both can have bpf progs attached at different points, but napi won't run
> when bpf prog is running, because bpf prog disables preemption.

Disabling preemption is not sufficient, it needs to have done
local_bh_disable(), which isn't unlikely given this is all networking
code.

Anyway, if you're sure that's all good, I'll trust you on that. It's
been a very _very_ long time since I looked at the networking code.