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

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Message-ID: <20190125023402.34a5k62furpdismi@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 24 Jan 2019 18:34:03 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 06:29:55PM -0800, Eric Dumazet wrote:
> 
> 
> On 01/24/2019 03:58 PM, Alexei Starovoitov wrote:
> > On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote:
> 
> >> 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.
> 
> 
> Speaking of NMI, how pcpu_freelist_push() and pop() can actually work ?
> 
> It seems bpf_get_stackid() can be called from NMI, and lockdep seems to complain loudly

it's a known false positive.
https://lkml.org/lkml/2018/7/25/756
and the same answer as before:
we're not going to penalize performance to shut up false positive.