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

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Date:   Fri, 25 Jan 2019 11:09:06 +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 Thu, Jan 24, 2019 at 03:58:59PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote:

> > So clearly this map stuff is shared between bpf proglets, otherwise
> > there would not be a need for locking. But what happens if one is from
> > task context and another from IRQ context?
> > 
> > I don't see a local_irq_save()/restore() anywhere. What avoids the
> > trivial lock inversion?
> 
> > and from NMI ...
> 
> progs are not preemptable and map syscall accessors have bpf_prog_active counters.
> So nmi/kprobe progs will not be running when syscall is running.
> Hence dead lock is not possible and irq_save is not needed.

What about the progs that run from SoftIRQ ? Since that bpf_prog_active
thing isn't inside BPF_PROG_RUN() what is to stop say:

   reuseport_select_sock()
     ...
       BPF_PROG_RUN()
         bpf_spin_lock()
        <IRQ>
	  ...
	  BPF_PROG_RUN()
	    bpf_spin_lock() // forever more

	</IRQ>

Unless you stick that bpf_prog_active stuff inside BPF_PROG_RUN itself,
I don't see how you can fundamentally avoid this happening (now or in
the future).