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SubjectRe: [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@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
> > >

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