Re: [PATCH 2/2] iio: trigger: Fix reference counting

From: Jonathan Cameron <jic23@kernel.org>
To: Lars-Peter Clausen <lars@metafoo.de>
Cc: Martin Fuzzey <mfuzzey@parkeon.com>,
	Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
	linux-iio@vger.kernel.org
Subject: Re: [PATCH 2/2] iio: trigger: Fix reference counting
Date: Thu, 28 Oct 2021 17:12:08 +0100	[thread overview]
Message-ID: <20211028171208.0edaa4e1@jic23-huawei> (raw)
In-Reply-To: <41bc22f4-071e-9ede-bce2-2d3c9cdf0344@metafoo.de>

On Thu, 28 Oct 2021 18:04:22 +0200
Lars-Peter Clausen <lars@metafoo.de> wrote:

> On 10/28/21 4:16 PM, Jonathan Cameron wrote:
> > On Sun, 24 Oct 2021 11:27:00 +0200
> > Lars-Peter Clausen <lars@metafoo.de> wrote:
> >  
> >> In viio_trigger_alloc() device_initialize() is used to set the initial
> >> reference count of the trigger to 1. Then another get_device() is called on
> >> trigger. This sets the reference count to 2 before the trigger is returned.
> >>
> >> iio_trigger_free(), which is the matching API to viio_trigger_alloc(),
> >> calls put_device() which decreases the reference count by 1. But the second
> >> reference count acquired in viio_trigger_alloc() is never dropped.
> >>
> >> As a result the iio_trigger_release() function is never called and the
> >> memory associated with the trigger is never freed.
> >>
> >> Since there is no reason for the trigger to start its lifetime with two
> >> reference counts just remove the extra get_device() in
> >> viio_trigger_alloc().
> >>
> >> Fixes: 5f9c035cae18 ("staging:iio:triggers. Add a reference get to the core for triggers.")
> >> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>  
> > I fully agree the current code is wrong, but we really should be using
> > device_put() in the error path after device_initialize() has been called.
> >
> > There are multiple places where we currently do this wrong in IIO but this particular
> > one looks like a local fix would be safe.
> > Worth doing that in the same patch at this one given it's all about reference
> > counting logic being wrong?  If not, we can do it as a separate follow up patch.  
> I already have that patch. Just waiting for this to be applied since it 
> has a dependency.

In that case, applied for this one to the fixes-togreg branch of iio.git.

Thanks,

Jonathan

> >
> > Jonathan
> >
> >  
> >> ---
> >> I'm a bit unsure about the fixes tag. I've looked at the IIO tree at the
> >> point when this was introduced and I believe it was incorrect even back
> >> then.
> >>
> >> But we also had a few drivers that directly assigned the indio_dev->trig
> >> without getting an extra reference. So these two bugs, one in the core, one
> >> in the drivers sort of even out. Except that iio_trigger_get() also gets a
> >> reference to the drivers module and iio_trigger_put() releases it again. So
> >> with the missing iio_trigger_get() there is still the problem that, even
> >> though the device references balance out, there is a module reference count
> >> imbalance.
> >> ---
> >>   drivers/iio/industrialio-trigger.c | 1 -
> >>   1 file changed, 1 deletion(-)
> >>
> >> diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
> >> index b23caa2f2aa1..93990ff1dfe3 100644
> >> --- a/drivers/iio/industrialio-trigger.c
> >> +++ b/drivers/iio/industrialio-trigger.c
> >> @@ -556,7 +556,6 @@ struct iio_trigger *viio_trigger_alloc(struct device *parent,
> >>   		irq_modify_status(trig->subirq_base + i,
> >>   				  IRQ_NOREQUEST | IRQ_NOAUTOEN, IRQ_NOPROBE);
> >>   	}
> >> -	get_device(&trig->dev);
> >>   
> >>   	return trig;
> >>     
> 
> 