linux-kernel - Re: [PATCH v2 27/52] x86, pci, amd-bus: Fix CPU hotplug callback registration

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Date:	Fri, 14 Feb 2014 10:35:03 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"paulus@...ba.org" <paulus@...ba.org>, oleg@...hat.com,
	Ingo Molnar <mingo@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Michel Lespinasse <walken@...gle.com>, ego@...ux.vnet.ibm.com,
	Russell King <linux@....linux.org.uk>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v2 27/52] x86, pci, amd-bus: Fix CPU hotplug callback registration

On Fri, Feb 14, 2014 at 12:55 AM, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> 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 amd-bus code in x86 by using this latter form of callback
> registration.
>
> Cc: Bjorn Helgaas <bhelgaas@...gle.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: linux-pci@...r.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

This looks fine to me.

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

I think it makes sense for you to merge it along with all your other
similar changes.  I don't foresee any conflicts with things in my PCI
tree.

> ---
>
>  arch/x86/pci/amd_bus.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/pci/amd_bus.c b/arch/x86/pci/amd_bus.c
> index a48be98..f05cbf0 100644
> --- a/arch/x86/pci/amd_bus.c
> +++ b/arch/x86/pci/amd_bus.c
> @@ -380,10 +380,13 @@ static int __init pci_io_ecs_init(void)
>         if (early_pci_allowed())
>                 pci_enable_pci_io_ecs();
>
> -       register_cpu_notifier(&amd_cpu_notifier);
> +       cpu_notifier_register_begin();
>         for_each_online_cpu(cpu)
>                 amd_cpu_notify(&amd_cpu_notifier, (unsigned long)CPU_ONLINE,
>                                (void *)(long)cpu);
> +       __register_cpu_notifier(&amd_cpu_notifier);
> +       cpu_notifier_register_done();
> +
>         pci_probe |= PCI_HAS_IO_ECS;
>
>         return 0;
>
--
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