linux-kernel - Re: [PATCH v2 6/6] hwmon: pmbus: use more devres helpers

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Message-ID: <432eb32c-3991-d001-29f9-e98389b9a7a6@roeck-us.net>
Date:   Thu, 2 Jul 2020 07:29:32 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jean Delvare <jdelvare@...e.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v2 6/6] hwmon: pmbus: use more devres helpers

On 7/2/20 5:44 AM, Greg Kroah-Hartman wrote:
> On Mon, Jun 29, 2020 at 08:50:08AM +0200, Bartosz Golaszewski wrote:
>> From: Bartosz Golaszewski <bgolaszewski@...libre.com>
>>
>> Shrink pmbus code by using devm_hwmon_device_register_with_groups()
>> and devm_krealloc() instead of their non-managed variants.
>>
>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>
>> ---
>>  drivers/hwmon/pmbus/pmbus_core.c | 28 +++++++++-------------------
>>  1 file changed, 9 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
>> index a420877ba533..225d0ac162c7 100644
>> --- a/drivers/hwmon/pmbus/pmbus_core.c
>> +++ b/drivers/hwmon/pmbus/pmbus_core.c
>> @@ -1022,9 +1022,9 @@ static int pmbus_add_attribute(struct pmbus_data *data, struct attribute *attr)
>>  {
>>  	if (data->num_attributes >= data->max_attributes - 1) {
>>  		int new_max_attrs = data->max_attributes + PMBUS_ATTR_ALLOC_SIZE;
>> -		void *new_attrs = krealloc(data->group.attrs,
>> -					   new_max_attrs * sizeof(void *),
>> -					   GFP_KERNEL);
>> +		void *new_attrs = devm_krealloc(data->dev, data->group.attrs,
>> +						new_max_attrs * sizeof(void *),
>> +						GFP_KERNEL);
> 
> dynamic sysfs attributes in a devm-allocated chunk of memory?  What
> could go wrong...
> 

You mean that the memory might be removed before the attributes
are removed ? Hmm, but that isn't different to the current implementation.
The hwmon device is removed first (removing the sysfs attributes),
followed by the kfree. Are you saying this is not safe ?
Pretty much all code which allocates memory for struct attribute
is doing the same, so that would be a problem throughout the kernel.

> Anyway, is this the only in-kernel user that you could find for this
> function?  If so, it feels like it's a lot of extra work for no real
> gain.
> 
And I was so happy that I'd be able to get rid of pmbus_do_remove()
subsequently. But then I can also use devm_add_action() and have it
call kfree(), with the same result. Given that, if hwmon would really
be the only user, we can live without it.

Guenter