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

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Message-ID: <CAPDyKFpbeLJUiB_xQbqDib+-8Q3AcJNVg+DuEcqmVGMbFdNxwA@mail.gmail.com>
Date: Wed, 21 May 2025 14:37:08 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Claudiu Beznea <claudiu.beznea@...on.dev>
Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>, Jonathan Cameron <jic23@...nel.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, dakr@...nel.org, 
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org, geert@...ux-m68k.org, 
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-iio@...r.kernel.org, bhelgaas@...gle.com
Subject: Re: [PATCH] driver core: platform: Use devres group to free driver
 probe resources

On Wed, 21 May 2025 at 07:41, Claudiu Beznea <claudiu.beznea@...on.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