linux-kernel - Re: [PATCH 2/8] x86/mtrr/main.c: Ask the first online CPU to save mtrr

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Message-ID: <4E8CAA05.9010200@linux.vnet.ibm.com>
Date:	Thu, 06 Oct 2011 00:33:33 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Fenghua Yu <fenghua.yu@...el.com>
CC:	Ingo Molnar <mingo@...e.hu>, srivatsa.bhat@...ux.vnet.ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	Tony Luck <tony.luck@...el.com>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Len Brown <lenb@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] x86/mtrr/main.c: Ask the first online CPU to save
 mtrr

Hi,

On 10/05/2011 10:09 PM, Fenghua Yu wrote:
> From: Fenghua Yu <fenghua.yu@...el.com>
> 
> Ask the first online CPU to save mtrr instead of asking BSP. BSP can be
> offlined when mtrr_save_state() is called.
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> ---
>  arch/x86/kernel/cpu/mtrr/main.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> index 6b96110..e18a4d3 100644
> --- a/arch/x86/kernel/cpu/mtrr/main.c
> +++ b/arch/x86/kernel/cpu/mtrr/main.c
> @@ -695,11 +695,13 @@ void mtrr_ap_init(void)
>  }
> 
>  /**
> - * Save current fixed-range MTRR state of the BSP
> + * Save current fixed-range MTRR state of the first cpu in cpu_online_mask.
>   */
>  void mtrr_save_state(void)
>  {
> -	smp_call_function_single(0, mtrr_save_fixed_ranges, NULL, 1);
> +	int first_cpu = cpumask_first(cpu_online_mask);

There is a race condition here as well. What if, at this point, the cpu
represented by first_cpu is taken offline (due to a cpu hotplug operation)?

> +	smp_call_function_single(first_cpu, mtrr_save_fixed_ranges, NULL, 1);
>  }
> 
>  void set_mtrr_aps_delayed_init(void)


-- 
Regards,
Srivatsa S. Bhat  <srivatsa.bhat@...ux.vnet.ibm.com>
Linux Technology Center,
IBM India Systems and Technology Lab
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