Re: [PATCH 1/3] HID: logitech-hidpp: use devres to manage FF private data

[Benjamin Tissoires]

On Fri, Oct 11, 2019 at 8:26 PM Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
>
> On Fri, Oct 11, 2019 at 04:52:04PM +0200, Benjamin Tissoires wrote:
> > Hi Andrey,
> >
> > On Mon, Oct 7, 2019 at 7:13 AM Andrey Smirnov <andrew.smirnov@xxxxxxxxx> wrote:
> > >
> > > To simplify resource management in commit that follows as well as to
> > > save a couple of extra kfree()s and simplify hidpp_ff_deinit() switch
> > > driver code to use devres to manage the life-cycle of FF private data.
> > >
> > > Signed-off-by: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
> > > Cc: Jiri Kosina <jikos@xxxxxxxxxx>
> > > Cc: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> > > Cc: Henrik Rydberg <rydberg@xxxxxxxxxxx>
> > > Cc: Sam Bazely <sambazley@xxxxxxxxxxxx>
> > > Cc: Pierre-Loup A. Griffais <pgriffais@xxxxxxxxxxxxxxxxx>
> > > Cc: Austin Palmer <austinp@xxxxxxxxxxxxxxxxx>
> > > Cc: linux-input@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Cc: stable@xxxxxxxxxxxxxxx
> >
> > This patch doesn't seem to fix any error, is there a reason to send it
> > to stable? (besides as a dependency of the rest of the series).
> >
> > > ---
> > >  drivers/hid/hid-logitech-hidpp.c | 53 +++++++++++++++++---------------
> > >  1 file changed, 29 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
> > > index 0179f7ed77e5..58eb928224e5 100644
> > > --- a/drivers/hid/hid-logitech-hidpp.c
> > > +++ b/drivers/hid/hid-logitech-hidpp.c
> > > @@ -2079,6 +2079,11 @@ static void hidpp_ff_destroy(struct ff_device *ff)
> > >         struct hidpp_ff_private_data *data = ff->private;
> > >
> > >         kfree(data->effect_ids);
> >
> > Is there any reasons we can not also devm alloc data->effect_ids?
> >
> > > +       /*
> > > +        * Set private to NULL to prevent input_ff_destroy() from
> > > +        * freeing our devres allocated memory
> >
> > Ouch. There is something wrong here: input_ff_destroy() calls
> > kfree(ff->private), when the data has not been allocated by
> > input_ff_create(). This seems to lack a little bit of symmetry.
>
> Yeah, ff and ff-memless essentially take over the private data assigned
> to them. They were done before devm and the lifetime of the "private"
> data pieces was tied to the lifetime of the input device to simplify
> error handling and teardown.

Yeah, that stealing of the pointer is not good :)
But OTOH, it helps

>
> Maybe we should clean it up a bit... I'm open to suggestions.

The problem I had when doing the review was that there is no easy way
to have a "devm_input_ff_create_()", because the way it's built is
already "devres-compatible": the destroy gets called by input core.

So I don't have a good answer to simplify in a transparent manner
without breaking the API.

>
> In this case maybe best way is to get rid of hidpp_ff_destroy() and not
> set ff->private and rely on devm to free the buffers. One can get to
> device private data from ff methods via input_get_drvdata() since they
> all (except destroy) are passed input device pointer.

Sounds like a good idea. However, it seems there might be a race when
removing the workqueue:
the workqueue gets deleted in hidpp_remove, when the input node will
be freed by devres, so after the call of hidpp_remove.

So we should probably keep hidpp_ff_destroy() to clean up the ff bits,
and instead move the content of hidpp_ff_deinit() into
hidpp_ff_destroy() so we ensure proper ordering.

Andrey, note that ensuring the workqueue gets freed after the call of
input_destroy_device is something that should definitively go into
stable as this is a potential race problem.

Cheers,
Benjamin