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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Fri, Oct 11, 2019 at 01:33:03PM -0700, Dmitry Torokhov wrote:
> On Fri, Oct 11, 2019 at 09:25:52PM +0200, Benjamin Tissoires wrote:
> > On Fri, Oct 11, 2019 at 8:26 PM Dmitry Torokhov
> > <dmitry.torokhov@gmail.com> 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@gmail.com> 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@gmail.com>
> > > > > Cc: Jiri Kosina <jikos@kernel.org>
> > > > > Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
> > > > > Cc: Henrik Rydberg <rydberg@bitmath.org>
> > > > > Cc: Sam Bazely <sambazley@fastmail.com>
> > > > > Cc: Pierre-Loup A. Griffais <pgriffais@valvesoftware.com>
> > > > > Cc: Austin Palmer <austinp@valvesoftware.com>
> > > > > Cc: linux-input@vger.kernel.org
> > > > > Cc: linux-kernel@vger.kernel.org
> > > > > Cc: stable@vger.kernel.org
> > > >
> > > > 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.
> 
> I do not think we want devm_input_ff_create() explicitly, I think the
> fact that you can "build up" an input device by allocating it, then
> adding slots, poller, ff support, etc, and input core cleans it up is
> all good. It is just the ownership if the driver-private data block is
> not very obvious and is not compatible with allocating via devm.
> 
> > 
> > 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.
> 
> Yeah, well, that is a common issue with mixing devm and normal resources
> (and workqueue here is that "normal" resource), and we should either:
> 
> - not use devm
> - use devm_add_action_or_reset() to work in custom actions that work
>   freeing of non-managed resources into devm flow.

Actually, there is a door #3: use system workqueue. After all the work
that Tejun done on workqueues it is very rare that one actually needs
a dedicated workqueue (as works usually execute on one if the system
worker threads that are shared with other workqueues anyway).

-- 
Dmitry