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

Ulf Hansson
2025-05-21T12:37:45Z

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.

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.

Kind regards
Uffe