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

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