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

Guo Ren
2022-09-30T12:28:24Z

On Tue, Sep 20, 2022 at 9:45 AM Guo Ren <guoren@kernel.org> wrote:
>
> On Mon, Sep 19, 2022 at 7:59 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Sun, Sep 18, 2022 at 11:52:41AM -0400, guoren@kernel.org wrote:
> > > From: Guo Ren <guoren@linux.alibaba.com>
> > >
> > > 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@linux.alibaba.com>
> > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > ---
> > > 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.
It's my bug in page_fault, here is the solution:

@@ -358,6 +358,8 @@ asmlinkage void noinstr do_page_fault(struct pt_regs *regs)

__do_page_fault(regs);

+ local_irq_disable();
+
irqentry_exit(regs, state);
}
NOKPROBE_SYMBOL(do_page_fault);

>
> >
> > >
> > > + 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

--
Best Regards
Guo Ren