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Date: Tue, 9 Oct 2018 21:16:50 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Subject: Re: [PATCH v2 3/3] powerpc: machine check interrupt is a
 non-maskable interrupt
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On Tue, 9 Oct 2018 09:36:18 +0000
Christophe Leroy <christophe.leroy@c-s.fr> wrote:

> On 10/09/2018 05:30 AM, Nicholas Piggin wrote:
> > On Tue, 9 Oct 2018 06:46:30 +0200
> > Christophe LEROY <christophe.leroy@c-s.fr> wrote:
> >  =20
> >> Le 09/10/2018 =C3=A0 06:32, Nicholas Piggin a =C3=A9crit=C2=A0: =20
> >>> On Mon, 8 Oct 2018 17:39:11 +0200
> >>> Christophe LEROY <christophe.leroy@c-s.fr> wrote:
> >>>     =20
> >>>> Hi Nick,
> >>>>
> >>>> Le 19/07/2017 =C3=A0 08:59, Nicholas Piggin a =C3=A9crit=C2=A0: =20
> >>>>> Use nmi_enter similarly to system reset interrupts. This uses NMI
> >>>>> printk NMI buffers and turns off various debugging facilities that
> >>>>> helps avoid tripping on ourselves or other CPUs.
> >>>>>
> >>>>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> >>>>> ---
> >>>>>     arch/powerpc/kernel/traps.c | 9 ++++++---
> >>>>>     1 file changed, 6 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/trap=
s.c
> >>>>> index 2849c4f50324..6d31f9d7c333 100644
> >>>>> --- a/arch/powerpc/kernel/traps.c
> >>>>> +++ b/arch/powerpc/kernel/traps.c
> >>>>> @@ -789,8 +789,10 @@ int machine_check_generic(struct pt_regs *regs)
> >>>>>    =20
> >>>>>     void machine_check_exception(struct pt_regs *regs)
> >>>>>     {
> >>>>> -	enum ctx_state prev_state =3D exception_enter();
> >>>>>     	int recover =3D 0;
> >>>>> +	bool nested =3D in_nmi();
> >>>>> +	if (!nested)
> >>>>> +		nmi_enter(); =20
> >>>>
> >>>> This alters preempt_count, then when die() is called
> >>>> in_interrupt() returns true allthough the trap didn't happen in
> >>>> interrupt, so oops_end() panics for "fatal exception in interrupt"
> >>>> instead of gently sending SIGBUS the faulting app. =20
> >>>
> >>> Thanks for tracking that down.
> >>>     =20
> >>>> Any idea on how to fix this ? =20
> >>>
> >>> I would say we have to deliver the sigbus by hand.
> >>>
> >>>       if ((user_mode(regs)))
> >>>           _exception(SIGBUS, regs, BUS_MCEERR_AR, regs->nip);
> >>>       else
> >>>           die("Machine check", regs, SIGBUS);
> >>>     =20
> >>
> >> And what about all the other things done by 'die()' ?
> >>
> >> And what if it is a kernel thread ?
> >>
> >> In one of my boards, I have a kernel thread regularly checking the HW,
> >> and if it gets a machine check I expect it to gently stop and the die
> >> notification to be delivered to all registered notifiers.
> >>
> >> Until before this patch, it was working well. =20
> >=20
> > I guess the alternative is we could check regs->trap for machine
> > check in the die test. Complication is having to account for MCE
> > in an interrupt handler.
> >=20
> >         if (in_interrupt()) {
> >                  if (!IS_MCHECK_EXC(regs) || (irq_count() - (NMI_OFFSET=
 + HARDIRQ_OFFSET)))
> >                      panic("Fatal exception in interrupt");
> >         }
> >=20
> > Something like that might work for you? We needs a ppc64 macro for the
> > MCE, and can probably add something like in_nmi_from_interrupt() for
> > the second part of the test. =20
>=20
> Don't know, I'm away from home on business trip so I won't be able to=20
> test anything before next week. However it looks more or less like a=20
> hack, doesn't it ?

I thought it seemed okay (with the right functions added). Actually it
could be a bit nicer to do this, then it works generally :

         if (in_interrupt()) {
                  if (!in_nmi() || in_nmi_from_interrupt())
                      panic("Fatal exception in interrupt");
         }

>=20
> What about the following ?

Hmm, in some ways maybe it's nicer. One complication is I would like the
same thing to be available for platform specific machine check
handlers, so then you need to pass is_in_interrupt to them. Which you
can do without any problem... But is it cleaner than the above?

I guess one advantage of yours is that a BUG somewhere in the NMI path
will panic the system. Or is that a disadvantage?

Thanks,
Nick


>=20
> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> index fd58749b4d6b..1f09033a5103 100644
> --- a/arch/powerpc/kernel/traps.c
> +++ b/arch/powerpc/kernel/traps.c
> @@ -208,7 +208,7 @@ static unsigned long oops_begin(struct pt_regs *regs)
>   NOKPROBE_SYMBOL(oops_begin);
>=20
>   static void oops_end(unsigned long flags, struct pt_regs *regs,
> -			       int signr)
> +		     int signr, bool is_in_interrupt)
>   {
>   	bust_spinlocks(0);
>   	add_taint(TAINT_DIE, LOCKDEP_NOW_UNRELIABLE);
> @@ -247,7 +247,7 @@ static void oops_end(unsigned long flags, struct=20
> pt_regs *regs,
>   		mdelay(MSEC_PER_SEC);
>   	}
>=20
> -	if (in_interrupt())
> +	if (is_in_interrupt)
>   		panic("Fatal exception in interrupt");
>   	if (panic_on_oops)
>   		panic("Fatal exception");
> @@ -288,7 +288,7 @@ static int __die(const char *str, struct pt_regs=20
> *regs, long err)
>   }
>   NOKPROBE_SYMBOL(__die);
>=20
> -void die(const char *str, struct pt_regs *regs, long err)
> +static void nmi_die(const char *str, struct pt_regs *regs, long err,=20
> bool is_in_interrupt)
>   {
>   	unsigned long flags;
>=20
> @@ -303,7 +303,13 @@ void die(const char *str, struct pt_regs *regs,=20
> long err)
>   	flags =3D oops_begin(regs);
>   	if (__die(str, regs, err))
>   		err =3D 0;
> -	oops_end(flags, regs, err);
> +	oops_end(flags, regs, err, is_in_interrupt);
> +}
> +NOKPROBE_SYMBOL(nmi_die);
> +
> +void die(const char *str, struct pt_regs *regs, long err)
> +{
> +	nmi_die(str, regs, err, in_interrupt());
>   }
>   NOKPROBE_SYMBOL(die);
>=20
> @@ -737,6 +743,7 @@ int machine_check_generic(struct pt_regs *regs)
>   void machine_check_exception(struct pt_regs *regs)
>   {
>   	int recover =3D 0;
> +	bool is_in_interrupt =3D in_interrupt();
>   	bool nested =3D in_nmi();
>   	if (!nested)
>   		nmi_enter();
> @@ -765,7 +772,7 @@ void machine_check_exception(struct pt_regs *regs)
>   	if (check_io_access(regs))
>   		goto bail;
>=20
> -	die("Machine check", regs, SIGBUS);
> +	nmi_die("Machine check", regs, SIGBUS, is_in_interrupt);
>=20
>   	/* Must die if the interrupt is not recoverable */
>   	if (!(regs->msr & MSR_RI))
>=20
>=20
> Thanks
> Christophe