linux-kernel - Re: [PATCH 10/15] iio: sx9310: Simplify error return handling

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Date:   Tue, 28 Jul 2020 16:06:42 -0600
From:   Daniel Campello <campello@...gle.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     LKML <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Enrico Granata <egranata@...omium.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio <linux-iio@...r.kernel.org>
Subject: Re: [PATCH 10/15] iio: sx9310: Simplify error return handling

On Tue, Jul 28, 2020 at 3:32 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Daniel Campello (2020-07-28 14:23:29)
> > On Tue, Jul 28, 2020 at 1:40 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Daniel Campello (2020-07-28 08:12:53)
> > > > @@ -368,13 +368,13 @@ static int sx9310_wait_for_sample(struct sx9310_data *data)
> > > >  static int sx9310_read_proximity(struct sx9310_data *data,
> > > >                                  const struct iio_chan_spec *chan, int *val)
> > > >  {
> > > > -       int ret = 0;
> > > > +       int ret;
> > > >         __be16 rawval;
> > > >
> > > >         mutex_lock(&data->mutex);
> > > >
> > > >         ret = sx9310_get_read_channel(data, chan->channel);
> > > > -       if (ret < 0)
> > > > +       if (ret)
> > > >                 goto out;
> > > >
> > > >         if (data->client->irq) {
> > > > @@ -394,11 +394,11 @@ static int sx9310_read_proximity(struct sx9310_data *data,
> > > >
> > > >         mutex_lock(&data->mutex);
> > > >
> > > > -       if (ret < 0)
> > > > +       if (ret)
> > > >                 goto out_disable_irq;
> > >
> > > Why is this condition checked after grabbing the mutex? Shouldn't it be
> > > checked before grabbing the mutex? Or is that supposed to be a
> > > mutex_unlock()?
> > We acquire the lock before jumping to out_disable_irq which is before
> > a mutex_unlock()
>
> Does this function need to hold the mutex lock around get/put_read_channel?
Yes, both get/put_read_channel and get/put_event_channel use
sx9310_update_chan_en which is updating data->chan_{read,event}
bitmaps.
> It drops the lock while waiting and then regrabs it which seems to
> imply that another reader could come in and try to get the channel again
> during the wait. So put another way, it may be simpler to shorten the
> lock area and then bail out of this function to a place where the lock
> isn't held already on the return path.