linux-kernel - Re: [PATCH 26/51] x86, oprofile, nmi: Fix CPU hotplug callback registration

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Date:	Tue, 11 Feb 2014 00:37:37 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, 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, Robert Richter <rric@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH 26/51] x86, oprofile, nmi: Fix CPU hotplug callback
 registration

Hi,

On Thu, Feb 06, 2014 at 03:39:22AM +0530, Srivatsa S. Bhat wrote:
> Fix the oprofile code in x86 by using this latter form of callback
> registration. But retain the calls to get/put_online_cpus(), since they
> also protect the variables 'nmi_enabled' and 'ctr_running'.

get/put_online_cpus() protect us against cpu_hotplug_begin/end(). The
latter is always nested inside cpu_maps_update_begin/end(), which we
are already using here. 

So what additional protection are we getting by retaining
get/put_online_cpus() ?

> By nesting
> get/put_online_cpus() *inside* cpu_maps_update_begin/done(), we avoid
> the ABBA deadlock possibility mentioned above.
> 
> Cc: Robert Richter <rric@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> ---
> 
>  arch/x86/oprofile/nmi_int.c |   15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/oprofile/nmi_int.c b/arch/x86/oprofile/nmi_int.c
> index 6890d84..85e5f6e 100644
> --- a/arch/x86/oprofile/nmi_int.c
> +++ b/arch/x86/oprofile/nmi_int.c
> @@ -494,14 +494,19 @@ static int nmi_setup(void)
>  	if (err)
>  		goto fail;
> 
> +	cpu_maps_update_begin();
> +
> +	/* Use get/put_online_cpus() to protect 'nmi_enabled' */
>  	get_online_cpus();
> -	register_cpu_notifier(&oprofile_cpu_nb);
>  	nmi_enabled = 1;
>  	/* make nmi_enabled visible to the nmi handler: */
>  	smp_mb();
>  	on_each_cpu(nmi_cpu_setup, NULL, 1);
> +	__register_cpu_notifier(&oprofile_cpu_nb);
>  	put_online_cpus();
> 
> +	cpu_maps_update_done();
> +
>  	return 0;
>  fail:
>  	free_msrs();
> @@ -512,12 +517,18 @@ static void nmi_shutdown(void)
>  {
>  	struct op_msrs *msrs;
> 
> +	cpu_maps_update_begin();
> +
> +	/* Use get/put_online_cpus() to protect 'nmi_enabled' & 'ctr_running' */
>  	get_online_cpus();
> -	unregister_cpu_notifier(&oprofile_cpu_nb);
>  	on_each_cpu(nmi_cpu_shutdown, NULL, 1);
>  	nmi_enabled = 0;
>  	ctr_running = 0;
> +	__unregister_cpu_notifier(&oprofile_cpu_nb);
>  	put_online_cpus();
> +
> +	cpu_maps_update_done();
> +
>  	/* make variables visible to the nmi handler: */
>  	smp_mb();
>  	unregister_nmi_handler(NMI_LOCAL, "oprofile");
> 

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