On Wed, Aug 19, 2020 at 05:32:50PM +0200, peterz@infradead.org wrote:

On Wed, Aug 19, 2020 at 08:39:13PM +1000, Alexey Kardashevskiy wrote:

quoted

quoted

or current upstream?

The upstream 18445bf405cb (13 hours old) also shows the problem. Yours
1/2 still fixes it.

Afaict that just reduces the window.

Isn't the problem that:

arch/powerpc/kernel/exceptions-64e.S

	START_EXCEPTION(perfmon);
	NORMAL_EXCEPTION_PROLOG(0x260, BOOKE_INTERRUPT_PERFORMANCE_MONITOR,
				PROLOG_ADDITION_NONE)
	EXCEPTION_COMMON(0x260)
	INTS_DISABLE
#	  RECONCILE_IRQ_STATE
#	    TRACE_DISABLE_INTS
#	      TRACE_WITH_FRAME_BUFFER(trace_hardirqs_off)
#
# but we haven't done nmi_enter() yet... whoopsy

	CHECK_NAPPING()
	addi	r3,r1,STACK_FRAME_OVERHEAD
	bl	performance_monitor_exception
#	 perf_irq()
#          perf_event_interrupt
#	     __perf_event_interrupt
#	      nmi_enter()



That is, afaict your entry code is buggered.

That is, patch 1/2 doesn't change the case:

	local_irq_enable()
	  trace_hardirqs_on()
	  <NMI>
	    trace_hardirqs_off()
	    ...
	    if (regs_irqs_disabled(regs)) // false
	      trace_hardirqs_on();
	  </NMI>
	  raw_local_irq_enable()

Where local_irq_enable() has done trace_hardirqs_on() and the NMI hits
and undoes it, but doesn't re-do it because the hardware state is still
disabled.

What's supposed to happen is:

	<NMI>
	  nmi_enter()
	  trace_hardirqs_off() // no-op, because in_nmi() (or previously because lockdep_off())
	  ...
	</NMI>