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

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Date:   Thu, 2 Jul 2020 15:06:24 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jean Delvare <jdelvare@...e.com>,
        linux-doc <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <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 Thu, Jul 2, 2020 at 2:44 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> 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...
>

So what *can* go wrong, which it couldn't before this patch? The
drivers in this directory kfree() this memory anyway on driver detach.
Using devm here is equivalent to the previous behavior - only that the
memory is freed after remove() not inside it.

> 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.
>

No. There are around 100 calls to krealloc() in drivers/. I assume
that at least half of these are called with an attached struct device.
I chose this driver, because it has a commit in its history that
explicitly says that it would use devm_krealloc() if it were available
(commit 85cfb3a83536 ("hwmon: (pmbus) Use krealloc to allocate
attribute memory"). I didn't want to spend a lot of time on converting
other users in case this patch gets rejected.

Bartosz