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

[Nicholas Piggin]

On Mon, 04 Dec 2017 16:09:57 +1100
Michael Ellerman <mpe at ellerman.id.au> wrote:

> 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?

I'm not really sure. A customer hit it, during either a stress test or
long running workload with lockdep irq tracing and perf running at the
same time. I don't have a lot more details but we might be able to get
some offline if necessary.

Thanks,
Nick