Re: [PATCH 1/2] devres: add devm_remove_action_nowarn()

[Alice Ryhl]

On Tue, Jan 7, 2025 at 11:05 AM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> On Mon, Jan 06, 2025 at 12:47:52PM +0100, Simona Vetter wrote:
> > On Fri, Jan 03, 2025 at 05:44:30PM +0100, Danilo Krummrich wrote:
> > > devm_remove_action() warns if the action to remove does not exist
> > > (anymore).
> > >
> > > The Rust devres abstraction, however, has a use-case to call
> > > devm_remove_action() at a point where it can't be guaranteed that the
> > > corresponding action hasn't been released yet.
> > >
> > > In particular, an instance of `Devres<T>` may be dropped after the
> > > action has been released. So far, `Devres<T>` worked around this by
> > > keeping the inner type alive.
> > >
> > > Hence, add devm_remove_action_nowarn(), which returns an error code if
> > > the action has been removed already.
> > >
> > > A subsequent patch uses devm_remove_action_nowarn() to remove the action
> > > when `Devres<T>` is dropped.
> > >
> > > Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> > > ---
> > >  drivers/base/devres.c  | 17 ++++++++++++-----
> > >  include/linux/device.h | 18 +++++++++++++++++-
> > >  2 files changed, 29 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> > > index 2152eec0c135..d59b8078fc33 100644
> > > --- a/drivers/base/devres.c
> > > +++ b/drivers/base/devres.c
> > > @@ -750,25 +750,32 @@ int __devm_add_action(struct device *dev, void (*action)(void *), void *data, co
> > >  EXPORT_SYMBOL_GPL(__devm_add_action);
> > >
> > >  /**
> > > - * devm_remove_action() - removes previously added custom action
> > > + * devm_remove_action_nowarn() - removes previously added custom action
> > >   * @dev: Device that owns the action
> > >   * @action: Function implementing the action
> > >   * @data: Pointer to data passed to @action implementation
> > >   *
> > >   * Removes instance of @action previously added by devm_add_action().
> > >   * Both action and data should match one of the existing entries.
> > > + *
> > > + * In contrast to devm_remove_action(), this function does not WARN() if no
> > > + * entry could have been found.
> >
> > I'd put a caution here that most likely, using this is a bad idea. Maybe
> > something like:
> >
> > "This should only be used if the action is contained in an object with
> > independent lifetime management, like the Devres rust abstraction.
> > Anywhere is the warning most likely indicates a driver bug."
>
> Yes, I thought of something similar too, but wasn't quite sure if it's needed.
> At least for me, if something has the postfix "nowarn", it already makes me
> wonder if I should really use it.
>
> I'll add a paragraph.
>
> >
> > At least I really can't come up with a reasonable design in a C driver
> > that would ever need this.
>
> I tried, but couldn't either. The only thing I could think of was a revocable
> thing in C.

Potentially if there are two cleanup paths that could run in parallel,
they could use this to avoid needing to synchronize which one removes
it?

Alice