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From:   Michael Kelley <mikelley@microsoft.com>
To:     Wei Liu <wei.liu@kernel.org>,
        Linux on Hyper-V List <linux-hyperv@vger.kernel.org>
CC:     "stable@kernel.org" <stable@kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] x86/hyperv: check cpu mask after interrupt has been
 disabled
Thread-Topic: [PATCH] x86/hyperv: check cpu mask after interrupt has been
 disabled
Thread-Index: AQHW44tO5NnOeOGnHk+aOh42E5R4cKoZV4Mw
Date:   Tue, 5 Jan 2021 18:20:05 +0000
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References: <20210105175043.28325-1-wei.liu@kernel.org>
In-Reply-To: <20210105175043.28325-1-wei.liu@kernel.org>
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From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, January 5, 2021 9:51 AM
>=20
> We've observed crashes due to an empty cpu mask in
> hyperv_flush_tlb_others.  Obviously the cpu mask in question is changed
> between the cpumask_empty call at the beginning of the function and when
> it is actually used later.
>=20
> One theory is that an interrupt comes in between and a code path ends up
> changing the mask. Move the check after interrupt has been disabled to
> see if it fixes the issue.
>=20
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> Cc: stable@kernel.org
> ---
>  arch/x86/hyperv/mmu.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>=20
> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
> index 5208ba49c89a..2c87350c1fb0 100644
> --- a/arch/x86/hyperv/mmu.c
> +++ b/arch/x86/hyperv/mmu.c
> @@ -66,11 +66,17 @@ static void hyperv_flush_tlb_others(const struct cpum=
ask *cpus,
>  	if (!hv_hypercall_pg)
>  		goto do_native;
>=20
> -	if (cpumask_empty(cpus))
> -		return;
> -
>  	local_irq_save(flags);
>=20
> +	/*
> +	 * Only check the mask _after_ interrupt has been disabled to avoid the
> +	 * mask changing under our feet.
> +	 */
> +	if (cpumask_empty(cpus)) {
> +		local_irq_restore(flags);
> +		return;
> +	}
> +
>  	flush_pcpu =3D (struct hv_tlb_flush **)
>  		     this_cpu_ptr(hyperv_pcpu_input_arg);
>=20
> --
> 2.20.1

Reviewed-by:  Michael Kelley <mikelley@microsoft.com>