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

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

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.

cheers

quoted

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 3320bcac7192..36878b6ee8b8 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -911,9 +911,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	beq	1f
 	rlwinm	r7,r7,0,~PACA_IRQ_HARD_DIS
 	stb	r7,PACAIRQHAPPENED(r13)
-1:	li	r0,0
-	stb	r0,PACASOFTIRQEN(r13);
-	TRACE_DISABLE_INTS
+1:
+#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_BUG)
+	/* The interrupt should not have soft enabled. */
+	lbz	r7,PACASOFTIRQEN(r13)
+1:	tdnei	r7,0
+	EMIT_BUG_ENTRY 1b,__FILE__,__LINE__,BUGFLAG_WARNING
+#endif
 	b	.Ldo_restore
 
 	/*

-- 
2.15.0