netdev - Re: [PATCH bpf-next 1/2] bpf: enable stackmap with build

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Message-ID: <20180502173027.GM12180@hirez.programming.kicks-ass.net>
Date:   Wed, 2 May 2018 19:30:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Networking <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: enable stackmap with build_id in nmi
 context

On Wed, May 02, 2018 at 04:48:32PM +0000, Song Liu wrote:
> > It's broken though, I've bet you've never actually ran this with lockdep
> > enabled for example.
> 
> I am not following here. I just run the new selftest with CONFIG_LOCKDEP on, 
> and got no warning for this. 

Weird, I would be expecting complaints about releasing an unheld lock.

nmi_enter(),nmi_exit() have lockdep_off(),lockdep_on() resp. Which means
that the down_trylock() will not be recorded. The up, which is done from
IRQ context, will not be so supressed and should hit
print_unlock_imbalance_bug().

> > Also, you set work->sem before you do trylock, if the trylock fails you
> > return early and keep work->sem set, which will thereafter always result
> > in irq_work_busy.
> 
> work->sem was set after down_read_trylock(). I guess you misread the patch?

Argh, yes indeed. Sorry.