linux-kernel - Re: [PATCH] hwmon: (ads7871) Fix ads7871

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Date:	Tue, 17 Aug 2010 08:15:59 -0700
From:	Paul Thomas <pthomas8589@...il.com>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Axel Lin <axel.lin@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	lm-sensors@...sensors.org
Subject: Re: [PATCH] hwmon: (ads7871) Fix ads7871_probe error path

On Tue, Aug 17, 2010 at 4:07 AM, Jean Delvare <khali@...ux-fr.org> wrote:
> Hi Alex,
>
> On Mon, 16 Aug 2010 09:31:17 +0800, Axel Lin wrote:
>> We need to call hwmon_device_unregister() if an error is detected after
>> sucessfully register hwmon device.
>>
>> Signed-off-by: Axel Lin <axel.lin@...il.com>
>> ---
>>  drivers/hwmon/ads7871.c |    4 +++-
>>  1 files changed, 3 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
>> index b300a20..303db92 100644
>> --- a/drivers/hwmon/ads7871.c
>> +++ b/drivers/hwmon/ads7871.c
>> @@ -201,11 +201,13 @@ static int __devinit ads7871_probe(struct spi_device *spi)
>>       we need to make sure we really have a chip*/
>>       if (val != ret) {
>>               err = -ENODEV;
>> -             goto error_remove;
>> +             goto error_unregister;
>>       }
>>
>>       return 0;
>>
>> +error_unregister:
>> +     hwmon_device_unregister(pdata->hwmon_dev);
>>  error_remove:
>>       sysfs_remove_group(&spi->dev.kobj, &ads7871_group);
>>  error_free:
>
> The bug is real, but I don't think the fix is correct. You should never
> have to unregister the hwmon device in the error path, because you
> should ensure the hardware is there and working _before_ you register
> the hwmon device.
>
> User-space could watch for hwmon devices being added or removed, and
> you don't want to trigger such events for a device which isn't going to
> work.
>
> So I would suggest reworking the order in which things are done,
> leaving hwmon_device_register() at the end of the function. This won't
> only fix the error path, this will also close a race window, as for the
> moment, the device is exposed to user-space _before_ it it properly
> initialized, which is wrong.
>
> Please send an updated patch and I'll be happy to apply it.
>
> --
> Jean Delvare
>

Should the kzalloc,  sysfs_create_group and hwmon_device_register all
stay in the same order just following the register check? I can send a
patch for that.

thanks,
Paul
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