Subject: Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and rcu_idle_exit noinstr

From: Frederic Weisbecker <[email-masked]>
Date: Mon, 16 May 2022 13:49:22 +0200

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 <[email-masked]>
> 
> From an RCU viewpoint:
> 
> Reviewed-by: Paul E. McKenney <[email-masked]>
> 
> [ 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/[email-masked]/

Thanks.

> 
> > ---
> >  arch/x86/kernel/process.c | 2 +-
> >  kernel/rcu/tree.c         | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index b370767f5b19..1345cb0124a6 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -720,7 +720,7 @@ void arch_cpu_idle_dead(void)
> >  /*
> >   * Called from the generic idle code.
> >   */
> > -void arch_cpu_idle(void)
> > +void noinstr arch_cpu_idle(void)
> >  {
> >  	x86_idle();
> >  }
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index a4b8189455d5..20d529722f51 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -896,7 +896,7 @@ static void noinstr rcu_eqs_exit(bool user)
> >   * If you add or remove a call to rcu_idle_exit(), be sure to test with
> >   * CONFIG_RCU_EQS_DEBUG=y.
> >   */
> > -void rcu_idle_exit(void)
> > +void noinstr rcu_idle_exit(void)
> >  {
> >  	unsigned long flags;
> >  
> > -- 
> > 2.35.3
> >