Re: [PATCH] powerpc/64s: fix may_hard_irq_enable for PMI soft masking

On Tue, 6 Feb 2018 15:30:43 +0530
Madhavan Srinivasan [off-list ref] wrote:

On Saturday 03 February 2018 12:47 PM, Nicholas Piggin wrote:

quoted

vThe soft IRQ masking code has to hard-disable interrupts in cases
where the exception is not cleared by the masked handler. External
interrupts used this approach for soft masking. Now recently PMU
interrupts do the same thing.

The soft IRQ masking code additionally allowed for interrupt handlers
to hard-enable interrupts after soft-disabling them. The idea is to
allow PMU interrupts through to profile interrupt handlers.

So when interrupts are being replayed when there is a pending
interrupt that requires hard-disabling, there is a test to prevent
those handlers from hard-enabling them if there is a pending external
interrupt. may_hard_irq_enable() handles this.

After f442d00480 ("powerpc/64s: Add support to mask perf interrupts
and replay them"), may_hard_irq_enable() could prematurely enable
MSR[EE] when a PMU exception exists, which would result in the
interrupt firing again while masked, and MSR[EE] being disabled again.

I haven't seen that this could cause a serious problem, but it's
more consistent to handle these soft-masked interrupts in the same
way. So introduce a define for all types of interrupts that require
MSR[EE] masking in their soft-disable handlers, and use that in
may_hard_irq_enable().

Cc: Madhavan Srinivasan <redacted>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>  

Yes nice catch. We could get into a messy state.
May be if I run my perf+kernbench longer, could get lucky.

I think we just get away with it, because I think the worst that
happens is a PMU exception will fire an interrupt again while it
is still soft-masked, during the interrupt replay code. It would
run its masked handler and set MSR[EE]=0 again and later be
replayed.

But this tidies things up and makes them consistent, and won't
cause any surprises if code changes in future. Thanks for the
review.

Thanks,
Nick