linux-kernel - Re: [PATCH 13/51] powerpc, sysfs: Fix CPU hotplug callback registration

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Date:	Fri, 14 Feb 2014 12:17:04 +0530
From:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de, akpm@...ux-foundation.org
CC:	mingo@...nel.org, paulmck@...ux.vnet.ibm.com, tj@...nel.org,
	walken@...gle.com, ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Olof Johansson <olof@...om.net>,
	Wang Dongsheng <dongsheng.wang@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 13/51] powerpc, sysfs: Fix CPU hotplug callback registration

On Thursday 06 February 2014 03:36 AM, Srivatsa S. Bhat 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_maps_update_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_maps_update_done();
> 
> 
> Fix the sysfs code in powerpc by using this latter form of callback
> registration.

Acked-by: Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>

> 
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
> Cc: Olof Johansson <olof@...om.net>
> Cc: Wang Dongsheng <dongsheng.wang@...escale.com>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> ---
> 
>  arch/powerpc/kernel/sysfs.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> index 97e1dc9..c29ad44 100644
> --- a/arch/powerpc/kernel/sysfs.c
> +++ b/arch/powerpc/kernel/sysfs.c
> @@ -975,7 +975,8 @@ static int __init topology_init(void)
>  	int cpu;
> 
>  	register_nodes();
> -	register_cpu_notifier(&sysfs_cpu_nb);
> +
> +	cpu_maps_update_begin();
> 
>  	for_each_possible_cpu(cpu) {
>  		struct cpu *c = &per_cpu(cpu_devices, cpu);
> @@ -999,6 +1000,11 @@ static int __init topology_init(void)
>  		if (cpu_online(cpu))
>  			register_cpu_online(cpu);
>  	}
> +
> +	__register_cpu_notifier(&sysfs_cpu_nb);
> +
> +	cpu_maps_update_done();
> +
>  #ifdef CONFIG_PPC64
>  	sysfs_create_dscr_default();
>  #endif /* CONFIG_PPC64 */
> 

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