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

Frederic Weisbecker
2022-05-16T11:49:34Z

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/

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
> >