Re: [PATCH] driver core: platform: Use devres group to free driver probe resources

Ulf Hansson
2025-05-22T10:32:39Z

On Wed, 21 May 2025 at 16:58, Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
>
> On Wed, May 21, 2025 at 02:37:08PM +0200, Ulf Hansson wrote:
> > On Wed, 21 May 2025 at 07:41, Claudiu Beznea <claudiu.beznea@tuxon.dev> wrote:
> > >
> > > Hi, Ulf,
> > >
> > > On 20.05.2025 15:09, Ulf Hansson wrote:
> > > > For example, even if the order is made correctly, suppose a driver's
> > > > ->remove() callback completes by turning off the resources for its
> > > > device and leaves runtime PM enabled, as it relies on devres to do it
> > > > some point later. Beyond this point, nothing would prevent userspace
> > > > for runtime resuming/suspending the device via sysfs.
> > >
> > > If I'm not wrong, that can't happen? The driver_sysfs_remove() is called
> > > before device_remove() (which calls the driver remove) is called, this
> > > being the call path:
> > >
> > > device_driver_detach() ->
> > > device_release_driver_internal() ->
> > > __device_release_driver() ->
> > > driver_sysfs_remove()
> > > // ...
> > > device_remove()
> > >
> > > And the driver_sysfs_remove() calls in the end __kernfs_remove() which
> > > looks to me like the place that actually drops the entries from sysfs, this
> > > being a call path for it:
> > >
> > > driver_sysfs_remove() ->
> > > sysfs_remove_link() ->
> > > kernfs_remove_by_name() ->
> > > kernfs_remove_by_name_ns() ->
> > > __kernfs_remove() ->
> > >
> > > activating the following line in __kernfs_remove():
> > >
> > > pr_debug("kernfs %s: removing\n", kernfs_rcu_name(kn));
> > >
> > > leads to the following prints when unbinding the watchdog device from its
> > > watchdog driver (attached to platform bus) on my board:
> > > https://p.fr33tux.org/935252
> >
> > Indeed this is a very good point you make! I completely overlooked
> > this fact, thanks a lot for clarifying this!
> >
> > However, my main point still stands.
> >
> > In the end, there is nothing preventing rpm_suspend|resume|idle() in
> > drivers/base/power/runtime.c from running (don't forget runtime PM is
> > asynchronous too) for the device in question. This could lead to that
> > a ->runtime_suspend|resume|idle() callback becomes executed at any
> > point in time, as long as we haven't called pm_runtime_disable() for
> > the device.
>
> So exactly the same may happen if you enter driver->remove() and
> something calls runtime API before pm_runtime_disable() is called.
> The driver has (as they should be doing currently) be prepared for this.
>
> >
> > That's why the devm_pm_runtime_enable() should be avoided as it simply
> > introduces a race-condition. Drivers need to be more careful and use
> > pm_runtime_enable|disable() explicitly to control the behaviour.
>
> You make it sound like we are dealing with some non-deterministic
> process, like garbage collector, where runtime disable done by devm
> happens at some unspecified point in the future. However we are dealing
> with very well defined order of operations, all happening within
> __device_release_driver() call. It is the same scope as when using
> manual pm_runtime_disable(). Just the order is wrong, that is it.

I understand that devres is deterministic, the order to manage things
is ofcourse specified how we use it during ->probe() etc. My apologies
if it has sounded different to you.

What I have been trying to say is, because how the runtime PM works,
drivers must be careful about calling pm_runtime_enable|disable() as
relying on the order from devres is in many cases not sufficient.

Kind regards
Uffe