netdev - Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and rcu_idle

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Date:   Tue, 17 May 2022 12:13:45 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and
 rcu_idle_exit noinstr

On Mon, May 16, 2022 at 01:49:22PM +0200, Frederic Weisbecker wrote:
> On Sun, May 15, 2022 at 09:25:35PM -0700, Paul E. McKenney wrote:
> > On Sun, May 15, 2022 at 10:36:52PM +0200, Jiri Olsa wrote:
> > > Making arch_cpu_idle and rcu_idle_exit noinstr. Both functions run
> > > in rcu 'not watching' context and if there's tracer attached to
> > > them, which uses rcu (e.g. kprobe multi interface) it will hit RCU
> > > warning like:
> > > 
> > >   [    3.017540] WARNING: suspicious RCU usage
> > >   ...
> > >   [    3.018363]  kprobe_multi_link_handler+0x68/0x1c0
> > >   [    3.018364]  ? kprobe_multi_link_handler+0x3e/0x1c0
> > >   [    3.018366]  ? arch_cpu_idle_dead+0x10/0x10
> > >   [    3.018367]  ? arch_cpu_idle_dead+0x10/0x10
> > >   [    3.018371]  fprobe_handler.part.0+0xab/0x150
> > >   [    3.018374]  0xffffffffa00080c8
> > >   [    3.018393]  ? arch_cpu_idle+0x5/0x10
> > >   [    3.018398]  arch_cpu_idle+0x5/0x10
> > >   [    3.018399]  default_idle_call+0x59/0x90
> > >   [    3.018401]  do_idle+0x1c3/0x1d0
> > > 
> > > The call path is following:
> > > 
> > > default_idle_call
> > >   rcu_idle_enter
> > >   arch_cpu_idle
> > >   rcu_idle_exit
> > > 
> > > The arch_cpu_idle and rcu_idle_exit are the only ones from above
> > > path that are traceble and cause this problem on my setup.
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > 
> > From an RCU viewpoint:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> > 
> > [ I considered asking for an instrumentation_on() in rcu_idle_exit(),
> > but there is no point given that local_irq_restore() isn't something
> > you instrument anyway. ]
> 
> So local_irq_save() in the beginning of rcu_idle_exit() is unsafe because
> it is instrumentable by the function (graph)  tracers and the irqsoff tracer.
> 
> Also it calls into lockdep that might make use of RCU.
> 
> That's why rcu_idle_exit() is not noinstr yet. See this patch:
> 
> https://lore.kernel.org/lkml/20220503100051.2799723-4-frederic@kernel.org/

I see, could we mark it at least with notrace meanwhile?

jirka