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

On Mon, 04 Dec 2017 16:09:57 +1100
Michael Ellerman [off-list ref] wrote:

Nicholas Piggin [off-list ref] writes:

quoted

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@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@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