linux-kernel - Re: NMI for ARC

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Date:   Thu, 29 Sep 2016 10:30:02 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Vineet Gupta <vgupta@...opsys.com>
Cc:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: NMI for ARC

On Sep 28, 2016 6:20 PM, "Vineet Gupta" <vgupta@...opsys.com> wrote:
>
> On 09/28/2016 03:26 PM, Andy Lutomirski wrote:
> >> Right, so what I think Vineet is asking is if we need to disable NMIs as
> >> > well, we cannot on x86 disable NMIs so no.
> >> >
> > The same argument works here, too: an NMI won't set TIF_NEED_RESCHED
> > without sending an IPI, so we can't miss a wakeup.
>
> But what exact wakeup miss are we taking about here. If intr were NOT disabled,
> how could this happen. Just trying to understand the need for "irqs-disabled" in
> resume_{user,kernel}_*
>
> The intr disabled before reg file restore makes complete sense though.

An interrupt after the TIF_NEED_RESCHED check could set
TIF_NEED_RESCHED (because it woke a waiting task), and the resume code
wouldn't notice.  No IPI would be sent because TIF_NEED_RESCHED on the
current task is supposed to be acted on in a timely manner without
one.

More severe problems could happen.  A signal could be queued or, on
x86, a return-to-usermode notifier could be set by KVM on a
preemptible kernel.  The latter, if missed, will bring down the whole
machine.