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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        "Guilherme G . Piccoli" <gpiccoli@igalia.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/2] x86/crash: Disable virt in core NMI crash handler
 to avoid double list_add
In-Reply-To: <Yn0WPBGGI4VcbM4S@google.com>
References: <20220511234332.3654455-1-seanjc@google.com>
 <20220511234332.3654455-2-seanjc@google.com> <87wnervxb7.ffs@tglx>
 <Yn0WPBGGI4VcbM4S@google.com>
Date:   Thu, 12 May 2022 17:48:58 +0200
Message-ID: <87k0aqvjit.ffs@tglx>
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On Thu, May 12 2022 at 14:14, Sean Christopherson wrote:
> On Thu, May 12, 2022, Thomas Gleixner wrote:
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index 4e3a839ae146..808d3e75fb2d 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -896,7 +896,11 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
>                 msecs--;
>         }
>  
> -       /* Leave the nmi callback set */
> +       /*
> +        * Leave the nmi callback set, shootdown is a one-time thing.  Clearing
> +        * the callback could result in a NULL pointer dereference if a CPU
> +        * (finally) responds after the timeout expires.
> +        */

Good point!