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

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Date:	Fri, 14 Feb 2014 11:29:20 +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,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Gleb Natapov <gleb@...nel.org>, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 10/52] arm, kvm: Fix CPU hotplug callback registration

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@...aro.org>
> Cc: Gleb Natapov <gleb@...nel.org>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: kvmarm@...ts.cs.columbia.edu
> Cc: kvm@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.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@...hat.com>
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