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

From: Thomas Gleixner <tglx@linutronix.de>
To: Dmitry Safonov <dima@arista.com>
Cc: linux-kernel@vger.kernel.org,
	"Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Prasanna Panchamukhi" <panchamukhi@arista.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Borislav Petkov" <bp@alien8.de>,
	"Cathy Avery" <cavery@redhat.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
	"Mohammed Gamal" <mmorsy@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Roman Kagan" <rkagan@virtuozzo.com>,
	"Sasha Levin" <sashal@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	devel@linuxdriverproject.org, kvm@vger.kernel.org,
	linux-hyperv@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCHv2] x86/hyperv: Hold cpus_read_lock() on assigning reenlightenment vector
Date: Fri, 28 Jun 2019 00:39:30 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1906280033510.32342@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190617163955.25659-1-dima@arista.com>

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