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[72.189.64.225]) by smtp.gmail.com with ESMTPSA id u11sm805404qtk.61.2020.10.23.06.13.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2020 06:13:11 -0700 (PDT) Date: Fri, 23 Oct 2020 09:12:56 -0400 From: William Breathitt Gray To: David Lechner Cc: jic23@kernel.org, kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com, patrick.havelange@essensium.com, fabrice.gasnier@st.com, mcoquelin.stm32@gmail.com, alexandre.torgue@st.com Subject: Re: [PATCH v5 1/5] counter: Internalize sysfs interface code Message-ID: <20201023131256.GA30908@shinobu> References: <157d1edf-feec-33b5-7ad5-94f99316ca6e@lechnology.com> <20201018144901.GB231549@shinobu> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="0F1p//8PRICkK4MW" Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org --0F1p//8PRICkK4MW Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Oct 20, 2020 at 10:38:43AM -0500, David Lechner wrote: > On 10/18/20 9:49 AM, William Breathitt Gray wrote: > > On Mon, Oct 12, 2020 at 09:15:00PM -0500, David Lechner wrote: > >> On 9/26/20 9:18 PM, William Breathitt Gray wrote: > >>> diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter= -core.c > >>> new file mode 100644 > >>> index 000000000000..987c6e8277eb > >>> --- /dev/null > >>> +++ b/drivers/counter/counter-core.c > >> > >> > >>> +/** > >>> + * counter_register - register Counter to the system > >>> + * @counter: pointer to Counter to register > >>> + * > >>> + * This function registers a Counter to the system. A sysfs "counter= " directory > >>> + * will be created and populated with sysfs attributes correlating w= ith the > >>> + * Counter Signals, Synapses, and Counts respectively. > >>> + */ > >>> +int counter_register(struct counter_device *const counter) > >>> +{ > >>> + struct device *const dev =3D &counter->dev; > >>> + int err; > >>> + > >>> + /* Acquire unique ID */ > >>> + counter->id =3D ida_simple_get(&counter_ida, 0, 0, GFP_KERNEL); > >>> + if (counter->id < 0) > >>> + return counter->id; > >>> + > >>> + /* Configure device structure for Counter */ > >>> + dev->type =3D &counter_device_type; > >>> + dev->bus =3D &counter_bus_type; > >>> + if (counter->parent) { > >>> + dev->parent =3D counter->parent; > >>> + dev->of_node =3D counter->parent->of_node; > >>> + } > >>> + dev_set_name(dev, "counter%d", counter->id); > >>> + device_initialize(dev);> + dev_set_drvdata(dev, counter); > >>> + > >>> + /* Add Counter sysfs attributes */ > >>> + err =3D counter_sysfs_add(counter); > >>> + if (err) > >>> + goto err_free_id; > >>> + > >>> + /* Add device to system */ > >>> + err =3D device_add(dev); > >>> + if (err) { > >>> + put_device(dev); > >>> + goto err_free_id; > >>> + } > >>> + > >>> + return 0; > >>> + > >>> +err_free_id: > >>> + /* get_device/put_device combo used to free managed resources */ > >>> + get_device(dev); > >>> + put_device(dev); > >> > >> I've never seen this in a driver before, so it makes me think this is > >> not the "right way" to do this. After device_initialize() is called, we > >> already should have a reference to dev, so only device_put() is needed. > >=20 > > I do admit this feels very hacky, but I'm not sure how to handle this > > more elegantly. The problem is that device_initialize() is not enough by > > itself -- it just initializes the structure, while device_add() > > increments the reference count via a call to get_device(). >=20 > add_device() also releases this reference by calling put_device() in all > return paths. The reference count is initialized to 1 in device_initializ= e(). >=20 > device_initialize > kobject_init > kobject_init_internal > kref_init You're right, I completely overlooked this: kref_init() initializes the reference count to 1. Therefore, the get_device() call is incorrect and should be be removed. > >=20 > > So let's assume counter_sysfs_add() fails and we go to err_free_id > > before device_add() is called; if we ignore get_device(), the reference > > count will be 0 at this point. We then call put_device(), which calls > > kobject_put(), kref_put(), and eventually refcount_dec_and_test(). > >=20 > > The refcount_dec_and_test() function returns 0 at this point because the > > reference count can't be decremented further (refcount is already 0), so > > release() is never called and we end up leaking all the memory we > > allocated in counter_sysfs_add(). > >=20 > > Is my logic correct here? >=20 > Not quite. I think you missed a few things I mentioned above. So we never > have a ref count of 0 until the very last call to put_device(). Ack. 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