Re: [PATCH v2 1/6] drivers/base/devres: Introduce devm_release_action()

[Dan Williams <dan.j.williams@intel.com>]

On Tue, May 14, 2019 at 12:12 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, May 07, 2019 at 04:56:05PM -0700, Dan Williams wrote:
> > The devm_add_action() facility allows a resource allocation routine to
> > add custom devm semantics. One such user is devm_memremap_pages().
> >
> > There is now a need to manually trigger devm_memremap_pages_release().
> > Introduce devm_release_action() so the release action can be triggered
> > via a new devm_memunmap_pages() api in a follow-on change.
> >
> > Cc: Logan Gunthorpe <logang@deltatee.com>
> > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > ---
> >  drivers/base/devres.c  |   24 +++++++++++++++++++++++-
> >  include/linux/device.h |    1 +
> >  2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> > index e038e2b3b7ea..0bbb328bd17f 100644
> > --- a/drivers/base/devres.c
> > +++ b/drivers/base/devres.c
> > @@ -755,10 +755,32 @@ void devm_remove_action(struct device *dev, void (*action)(void *), void *data)
> >
> >       WARN_ON(devres_destroy(dev, devm_action_release, devm_action_match,
> >                              &devres));
> > -
> >  }
> >  EXPORT_SYMBOL_GPL(devm_remove_action);
> >
> > +/**
> > + * devm_release_action() - release previously added custom action
> > + * @dev: Device that owns the action
> > + * @action: Function implementing the action
> > + * @data: Pointer to data passed to @action implementation
> > + *
> > + * Releases and removes instance of @action previously added by
> > + * devm_add_action().  Both action and data should match one of the
> > + * existing entries.
> > + */
> > +void devm_release_action(struct device *dev, void (*action)(void *), void *data)
> > +{
> > +     struct action_devres devres = {
> > +             .data = data,
> > +             .action = action,
> > +     };
> > +
> > +     WARN_ON(devres_release(dev, devm_action_release, devm_action_match,
> > +                            &devres));
>
> What does WARN_ON help here?  are we going to start getting syzbot
> reports of this happening?

Hopefully, yes, if developers misuse the api they get a loud
notification similar to devm_remove_action() misuse.

> How can this fail?

It's a catch to make sure that @dev actually has a live devres
resource that can be found via @action and @data.