linux-kernel - Re: [PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online

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Date:	Fri, 3 Aug 2012 15:56:40 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Silas Boyd-Wickizer <sbw@....edu>
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Harald Welte <laforge@...nezx.org>, x86@...nel.org
Subject: Re: [PATCH 3/3] Use get_online_cpus to avoid races involving 
 for_each_online_cpu

Hi Silas,

On Thu, 2 Aug 2012 17:07:08 -0700, Silas Boyd-Wickizer wrote:
> via_cputemp_init in drivers/hwmon/via-cputemp.c loops with
> for_each_online_cpu, adding platform_devices, then calls
> register_hotcpu_notifier.  If a CPU is offlined between the loop and
> register_hotcpu_notifier, then later onlined, via_cputemp_device_add
> will attempt to platform devices with the same ID.

Missing word in this last sentence.

> 
> This fix surrounds for_each_online_cpu and register_hotcpu_notifier
> with get_online_cpus+put_online_cpus.
> 
> Build tested.

Thanks for reporting and for the fix. Two questions:

What about via_cputemp_exit()? While less obvious, I suspect it is racy
too. The notifier is unregistered first. If a CPU gets offline before
the devices are removed, we will have a device pointing to a
non-existent CPU for a short time. I think we should play it safe and
use get/put_online_cpus() there too, as I seem to understand it
guarantees CPUs can't go offline when we wouldn't handle that event
properly. Alternatively, unregistering the platform driver first might
close the race.

Secondly, drivers/hwmon/coretemp.c is very similar to via-cputemp.c, so
I think it needs the exact same fix(es).

> 
> Signed-off-by: Silas Boyd-Wickizer <sbw@....edu>
> ---
>  drivers/hwmon/via-cputemp.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/hwmon/via-cputemp.c b/drivers/hwmon/via-cputemp.c
> index 8689664..9ad07c3 100644
> --- a/drivers/hwmon/via-cputemp.c
> +++ b/drivers/hwmon/via-cputemp.c
> @@ -328,6 +328,7 @@ static int __init via_cputemp_init(void)
>  	if (err)
>  		goto exit;
>  
> +	get_online_cpus();
>  	for_each_online_cpu(i) {
>  		struct cpuinfo_x86 *c = &cpu_data(i);
>  
> @@ -347,12 +348,14 @@ static int __init via_cputemp_init(void)
>  
>  #ifndef CONFIG_HOTPLUG_CPU
>  	if (list_empty(&pdev_list)) {
> +		put_online_cpus();
>  		err = -ENODEV;
>  		goto exit_driver_unreg;
>  	}
>  #endif
>  
>  	register_hotcpu_notifier(&via_cputemp_cpu_notifier);
> +	put_online_cpus();
>  	return 0;
>  
>  #ifndef CONFIG_HOTPLUG_CPU


-- 
Jean Delvare
--
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