Subject: Re: [PATCH 2/4] powerpc/64: do not trace irqs-off at interrupt return to soft-disabled context

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 04 Dec 2017 10:12:02 -0600

On Mon, 2017-12-04 at 16:09 +1100, Michael Ellerman wrote:
> Nicholas Piggin <[email-masked]> writes:
> 
> > When an interrupt is returning to a soft-disabled context (which can
> > happen for non-maskable interrupts or synchronous interrupts), it goes
> > through the motions of soft-disabling again, including calling
> > TRACE_DISABLE_INTS (i.e., trace_hardirqs_off()).
> > 
> > This is not necessary, because we must already be soft-disabled in the
> > interrupt context, it also may be causing crashes in the irq tracing
> > code to re-enter as an nmi. Replace it with a warning to ensure that
> > soft-interrupts are still disabled.
> > 
> > Signed-off-by: Nicholas Piggin <[email-masked]>
> > ---
> >  arch/powerpc/kernel/entry_64.S | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> So this patch is the core of the bug fix I gather.
> 
> Git blames says:
> 
>   Fixes: 7c0482e3d055 ("powerpc/irq: Fix another case of lazy IRQ state getting out of sync")
>   Cc: [email-masked] # v3.4+
> 
> But I'm wondering how this has been broken that long without us
> noticing? You hit it doing some sort of perf stress test I think - so is
> it just that we've never pushed hard enough? Or did something change to
> expose this? Or we're just not sure?

We have some traps that do local_irq_enable ... you may want to double
check instruction emu, page faults, alignment etc... I wouldn't be
surprised if we have case where an interrupt "returns" soft enabled.

> cheers
> 
> > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> > index 3320bcac7192..36878b6ee8b8 100644
> > --- a/arch/powerpc/kernel/entry_64.S
> > +++ b/arch/powerpc/kernel/entry_64.S
> > @@ -911,9 +911,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
> >  	beq	1f
> >  	rlwinm	r7,r7,0,~PACA_IRQ_HARD_DIS
> >  	stb	r7,PACAIRQHAPPENED(r13)
> > -1:	li	r0,0
> > -	stb	r0,PACASOFTIRQEN(r13);
> > -	TRACE_DISABLE_INTS
> > +1:
> > +#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_BUG)
> > +	/* The interrupt should not have soft enabled. */
> > +	lbz	r7,PACASOFTIRQEN(r13)
> > +1:	tdnei	r7,0
> > +	EMIT_BUG_ENTRY 1b,__FILE__,__LINE__,BUGFLAG_WARNING
> > +#endif
> >  	b	.Ldo_restore
> >  
> >  	/*
> > -- 
> > 2.15.0