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SubjectRe: [PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu
On Fri, Aug 03, 2012 at 03:56:40PM +0200, Jean Delvare wrote:
> 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.

Fixed.

>
> >
> > 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.

I added get/put_online_cpus().

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

Indeed -- I'll submit a fix for this.

Silas

>
> >
> > Signed-off-by: Silas Boyd-Wickizer <sbw@mit.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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