[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:

> 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.

No worries.

I'll put it in and add it to my maybe-for-stable list. We can let it
soak a bit and then send it to stable if we think it's necessary and has
no issues.

cheers