linux-kernel - Re: [PATCHv2] x86/hyperv: Hold cpus_read_lock() on assigning reenlightenment vector

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Date:   Fri, 28 Jun 2019 00:39:30 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Safonov <dima@...sta.com>
cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Prasanna Panchamukhi <panchamukhi@...sta.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Cathy Avery <cavery@...hat.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Sasha Levin <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv2] x86/hyperv: Hold cpus_read_lock() on assigning
 reenlightenment vector

On Mon, 17 Jun 2019, Dmitry Safonov wrote:
> @@ -196,7 +196,16 @@ void set_hv_tscchange_cb(void (*cb)(void))
>  	/* Make sure callback is registered before we write to MSRs */
>  	wmb();
>  
> +	/*
> +	 * As reenlightenment vector is global, there is no difference which
> +	 * CPU will register MSR, though it should be an online CPU.
> +	 * hv_cpu_die() callback guarantees that on CPU teardown
> +	 * another CPU will re-register MSR back.
> +	 */
> +	cpus_read_lock();
> +	re_ctrl.target_vp = hv_vp_index[raw_smp_processor_id()];
>  	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
> +	cpus_read_unlock();

Should work

>  	wrmsrl(HV_X64_MSR_TSC_EMULATION_CONTROL, *((u64 *)&emu_ctrl));
>  }
>  EXPORT_SYMBOL_GPL(set_hv_tscchange_cb);
> @@ -239,6 +248,7 @@ static int hv_cpu_die(unsigned int cpu)
>  
>  	rdmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
>  	if (re_ctrl.target_vp == hv_vp_index[cpu]) {
> +		lockdep_assert_cpus_held();

So you're not trusting the hotplug core code to hold the lock when it
brings a CPU down and invokes that callback? Come on

>  		/* Reassign to some other online CPU */
>  		new_cpu = cpumask_any_but(cpu_online_mask, cpu);

Thanks,

	tglx