Re: [PATCH v2 10/52] arm, kvm: Fix CPU hotplug callback registration

[Paolo Bonzini <pbonzini@redhat.com>]

Il 14/02/2014 08:51, 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 arm by using this latter form of callback registration.
>
> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> Cc: Gleb Natapov <gleb@kernel.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: kvmarm@lists.cs.columbia.edu
> Cc: kvm@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> ---
>
>  arch/arm/kvm/arm.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 1d8248e..147b917 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -1050,21 +1050,26 @@ int kvm_arch_init(void *opaque)
>  		}
>  	}
>
> +	cpu_notifier_register_begin();
> +
>  	err = init_hyp_mode();
>  	if (err)
>  		goto out_err;
>
> -	err = register_cpu_notifier(&hyp_init_cpu_nb);
> +	err = __register_cpu_notifier(&hyp_init_cpu_nb);
>  	if (err) {
>  		kvm_err("Cannot register HYP init CPU notifier (%d)\n", err);
>  		goto out_err;
>  	}
>
> +	cpu_notifier_register_done();
> +
>  	hyp_cpu_pm_init();
>
>  	kvm_coproc_table_init();
>  	return 0;
>  out_err:
> +	cpu_notifier_register_done();
>  	return err;
>  }
>
>

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