Re: [PATCH V5 06/11] entry: Prevent DEBUG_PREEMPT warning

[Guo Ren]

On Mon, Sep 19, 2022 at 7:59 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sun, Sep 18, 2022 at 11:52:41AM -0400, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > When DEBUG_PREEMPT=y,
> >       exit_to_user_mode_prepare
> >       ->tick_nohz_user_enter_prepare
> >         ->tick_nohz_full_cpu(smp_processor_id())
> >           ->smp_processor_id()
> >             ->debug_smp_processor_id()
> >               ->check preempt_count() then:
> >
> > [    5.717610] BUG: using smp_processor_id() in preemptible [00000000]
> > code: S20urandom/94
> > [    5.718111] caller is debug_smp_processor_id+0x24/0x38
> > [    5.718417] CPU: 1 PID: 94 Comm: S20urandom Not tainted
> > 6.0.0-rc3-00010-gfd0a0d619c63-dirty #238
> > [    5.718886] Hardware name: riscv-virtio,qemu (DT)
> > [    5.719136] Call Trace:
> > [    5.719281] [<ffffffff800055fc>] dump_backtrace+0x2c/0x3c
> > [    5.719566] [<ffffffff80ae6cb0>] show_stack+0x44/0x5c
> > [    5.720023] [<ffffffff80aee870>] dump_stack_lvl+0x74/0xa4
> > [    5.720557] [<ffffffff80aee8bc>] dump_stack+0x1c/0x2c
> > [    5.721033] [<ffffffff80af65c0>]
> > check_preemption_disabled+0x104/0x108
> > [    5.721538] [<ffffffff80af65e8>] debug_smp_processor_id+0x24/0x38
> > [    5.722001] [<ffffffff800aee30>] exit_to_user_mode_prepare+0x48/0x178
> > [    5.722355] [<ffffffff80af5bf4>] irqentry_exit_to_user_mode+0x18/0x30
> > [    5.722685] [<ffffffff80af5c70>] irqentry_exit+0x64/0xa4
> > [    5.722953] [<ffffffff80af52f4>] do_page_fault+0x1d8/0x544
> > [    5.723291] [<ffffffff80003310>] ret_from_exception+0x0/0xb8
> >
> > (Above is found in riscv platform with generic_entry)
> >
> > The smp_processor_id() needs irqs disable or preempt_disable, so use
> > preempt dis/in protecting the tick_nohz_user_enter_prepare().
> >
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > ---
> >  kernel/entry/common.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index 063068a9ea9b..36e4cd50531c 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -194,8 +194,10 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
> >
> >       lockdep_assert_irqs_disabled();
>
>     Observe ^^^^
Thanks! I would enable PROVE_LOCKING for test.

>
> >
> > +     preempt_disable();
> >       /* Flush pending rcuog wakeup before the last need_resched() check */
> >       tick_nohz_user_enter_prepare();
> > +     preempt_enable();
>
> This makes no sense; if IRQs are disabled, check_preemption_disabled()
> should bail early per:
>
>         if (irqs_disabled())
>                 goto out;
Ditto.


-- 
Best Regards
 Guo Ren