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Date:	Fri, 14 Feb 2014 11:29:39 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	paulus@...ba.org, oleg@...hat.com, mingo@...nel.org,
	rusty@...tcorp.com.au, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
CC:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Gleb Natapov <gleb@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2 29/52] x86, kvm: Fix CPU hotplug callback registration

Il 14/02/2014 08:55, Srivatsa S. Bhat ha scritto:
> Subsystems that want to register CPU hotplug callbacks, as well as perform
> initialization for the CPUs that are already online, often do it as shown
> below:
>
> 	get_online_cpus();
>
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
>
> 	register_cpu_notifier(&foobar_cpu_notifier);
>
> 	put_online_cpus();
>
> This is wrong, since it is prone to ABBA deadlocks involving the
> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
> with CPU hotplug operations).
>
> Instead, the correct and race-free way of performing the callback
> registration is:
>
> 	cpu_notifier_register_begin();
>
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
>
> 	/* Note the use of the double underscored version of the API */
> 	__register_cpu_notifier(&foobar_cpu_notifier);
>
> 	cpu_notifier_register_done();
>
>
> Fix the kvm code in x86 by using this latter form of callback registration.
>
> Cc: Gleb Natapov <gleb@...nel.org>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: kvm@...r.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> ---
>
>  arch/x86/kvm/x86.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 39c28f09..0166923 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5365,7 +5365,8 @@ static void kvm_timer_init(void)
>  	int cpu;
>
>  	max_tsc_khz = tsc_khz;
> -	register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
> +
> +	cpu_notifier_register_begin();
>  	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
>  #ifdef CONFIG_CPU_FREQ
>  		struct cpufreq_policy policy;
> @@ -5382,6 +5383,10 @@ static void kvm_timer_init(void)
>  	pr_debug("kvm: max_tsc_khz = %ld\n", max_tsc_khz);
>  	for_each_online_cpu(cpu)
>  		smp_call_function_single(cpu, tsc_khz_changed, NULL, 1);
> +
> +	__register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
> +	cpu_notifier_register_done();
> +
>  }
>
>  static DEFINE_PER_CPU(struct kvm_vcpu *, current_vcpu);
>

Acked-by: Paolo Bonzini <pbonzini@...hat.com>
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