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

Benjamin Herrenschmidt [off-list ref] writes:

On Mon, 2017-12-04 at 16:09 +1100, Michael Ellerman wrote:

quoted

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?

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.

AFAICT those all check that they're coming from a soft-enabled context:

	if (!arch_irq_disabled_regs(regs))
		local_irq_enable();


And the code Nick is patching is the case where we're returning to a
soft-disabled context. So I think the patch is good.

cheers