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Message-ID: <f600d458-49dd-4c83-a40e-982b66c42684@paulmck-laptop>
Date: Thu, 12 Oct 2023 03:45:15 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, clm@...com,
tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, hpa@...or.com
Subject: Re: [PATCH RFC x86/nmi] Fix out-of-order nesting checks
On Thu, Oct 12, 2023 at 08:37:25AM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@...nel.org> wrote:
>
> > The ->idt_seq and ->recv_jiffies variables added by commit 1a3ea611fc10
> > ("x86/nmi: Accumulate NMI-progress evidence in exc_nmi()") place
> > the exit-time check of the bottom bit of ->idt_seq after the
> > this_cpu_dec_return() that re-enables NMI nesting. This can result in
> > the following sequence of events on a given CPU in kernels built with
> > CONFIG_NMI_CHECK_CPU=y:
> >
> > o An NMI arrives, and ->idt_seq is incremented to an odd number.
> > In addition, nmi_state is set to NMI_EXECUTING==1.
> >
> > o The NMI is processed.
> >
> > o The this_cpu_dec_return(nmi_state) zeroes nmi_state and returns
> > NMI_EXECUTING==1, thus opting out of the "goto nmi_restart".
> >
> > o Another NMI arrives and ->idt_seq is incremented to an even
> > number, triggering the warning. But all is just fine, at least
> > assuming we don't get so many closely spaced NMIs that the stack
> > overflows or some such.
> >
> > Experience on the fleet indicates that the MTBF of this false positive
> > is about 70 years. Or, for those who are not quite that patient, the
> > MTBF appears to be about one per week per 4,000 systems.
> >
> > Fix this false-positive warning by moving the "nmi_restart" label before
> > the initial ->idt_seq increment/check and moving the this_cpu_dec_return()
> > to follow the final ->idt_seq increment/check. This way, all nested NMIs
> > that get past the NMI_NOT_RUNNING check get a clean ->idt_seq slate.
> > And if they don't get past that check, they will set nmi_state to
> > NMI_LATCHED, which will cause the this_cpu_dec_return(nmi_state)
> > to restart.
>
> This looks like a sensible fix: the warning should obviously be atomic wrt.
> the no-nesting region. I've applied your fix to tip:x86/irq, as it doesn't
> seem urgent enough with a MTBF of 70 years to warrant tip:x86/urgent handling. ;-)
Works for me! ;-)
And thank you!
Thanx, Paul
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