linux-kernel - Re: [PATCH v6 21/29] x86/nmi: Add an NMI

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Date:   Mon, 09 May 2022 15:59:40 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        x86@...nel.org
Cc:     Tony Luck <tony.luck@...el.com>, Andi Kleen <ak@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Subject: Re: [PATCH v6 21/29] x86/nmi: Add an NMI_WATCHDOG NMI handler category

On Thu, May 05 2022 at 17:00, Ricardo Neri wrote:
> Add a NMI_WATCHDOG as a new category of NMI handler. This new category
> is to be used with the HPET-based hardlockup detector. This detector
> does not have a direct way of checking if the HPET timer is the source of
> the NMI. Instead, it indirectly estimates it using the time-stamp counter.
>
> Therefore, we may have false-positives in case another NMI occurs within
> the estimated time window. For this reason, we want the handler of the
> detector to be called after all the NMI_LOCAL handlers. A simple way
> of achieving this with a new NMI handler category.
>
> @@ -379,6 +385,10 @@ static noinstr void default_do_nmi(struct pt_regs *regs)
>  	}
>  	raw_spin_unlock(&nmi_reason_lock);
>  
> +	handled = nmi_handle(NMI_WATCHDOG, regs);
> +	if (handled == NMI_HANDLED)
> +		goto out;
> +

How is this supposed to work reliably?

If perf is active and the HPET NMI and the perf NMI come in around the
same time, then nmi_handle(LOCAL) can swallow the NMI and the watchdog
won't be checked. Because MSI is strictly edge and the message is only
sent once, this can result in a stale watchdog, no?

Thanks,

        tglx