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

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> 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 <npiggin at gmail.com>
> ---
>  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: stable at vger.kernel.org # 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?

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