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

["Paul E. McKenney" <paulmck@kernel.org>]

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@kernel.org>
> > 
> > From an RCU viewpoint:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@kernel.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/

Ah, I should have looked at the context-tracking series again!

And I have to ask...  How much debugging capability are we really losing
by continuing to use the raw versions of local_irq_{save,restore}()?

							Thanx, Paul

> 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
> > >