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

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Message-ID: <20190128083508.GA28878@hirez.programming.kicks-ass.net>
Date:   Mon, 28 Jan 2019 09:35:08 +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 09:31:23AM +0100, Peter Zijlstra wrote:
> On Fri, Jan 25, 2019 at 03:42:43PM -0800, Alexei Starovoitov wrote:
> > On Fri, Jan 25, 2019 at 10:10:57AM +0100, Peter Zijlstra wrote:
> > > On Thu, Jan 24, 2019 at 03:58:59PM -0800, Alexei Starovoitov wrote:
> 
> > >  > nmi checks for bpf_prog_active==0. See bpf_overflow_handler.
> 
> > > yuck yuck yuck.. That's horrific :-( That means the whole BPF crud is
> > > unreliable and events can go randomly missing.
> > 
> > bpf_prog_active is the mechanism to workaround non-reentrant pieces of the kernel.
> 
> 'the kernel' or 'bpf' ?
> 
> perf has a recursion counter per context (task,softirq,hardirq,nmi) and
> that ensures that perf doesn't recurse in on itself while allowing the
> nesting of these contexts.
> 
> But if BPF itself is not able to deal with such nesting that won't work
> of course.

Ooh, later you say:

> Also we allow tracing progs to nest with networking progs.

Which seems to suggest BPF itself can suppord (limited) nesting.

See: kernel/events/internal.h:get_recursion_context()