[PATCH v2 10/14] arm64/nmi: Manage masking for superpriority interrupts along with DAIF

[Mark Brown]

On Thu, Dec 08, 2022 at 06:19:02PM +0100, Lorenzo Pieralisi wrote:

> I think I found a nasty spot. We are currently not handling ALLINT in
> arch_local_irq_enable/disable(). The issue I am facing is that we might
> end up preempting in IRQ context with ALLINT set in the exception path
> - arm64_preempt_schedule_irq() - which means we are running with all
> IRQs masked (that's normal; what's not normal is that local_irq_enable()
> does not clear ALLINT, see below).

Right, and handling ALLINT in arch_local_irq_enable/disable() isn't
exactly ideal since it means that whenever we mask interrupts we also
mask NMIs which somewhat reduces the value.

> When we schedule (preempt_schedule_irq()) we do require a
> local_irq_enable() to enable IRQs; ALLINT is still set, so
> local_irq_enable() does not do what is expected so we are calling
> __schedule() with IRQs disabled, which does not seem right.

> Now we need to debate what the fix for this can be but nonetheless
> it is something to be addressed.

A first pass suggests that we should be handling this like we do for
other preemptions and returning early from arm64_preempt_schedule_irq()
if ALLINT is masked.  If we are handling a regular IRQ then ALLINT will
be unmasked and we'll call into preempt_schedule_irq(), if we're
handling a NMI then ALLINT will still be masked so we don't attempt to
schedule.  I've pushed out a change which does this but not yet properly
tested it.

> Clearing and setting ALLINT in arch_local_irq_enable()/disable()
> seems to solve the issue (now I moved on to debugging something
> else, will post the outcome here because this fix does not seem
> to fix the issue completely or I am hitting another bug).

Do you have any specifics on how you're seeing problems?  You did
mention boot stalls offline but I've not been able to to reproduce this
locally in a way that I can identify (based on your mail now I've made
sure I've got preemption enabled).
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