From: William Breathitt Gray <vilhelm.gray@gmail.com>
To: David Lechner <david@lechnology.com>
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
Date: Fri, 23 Oct 2020 09:12:56 -0400 [thread overview]
Message-ID: <20201023131256.GA30908@shinobu> (raw)
In-Reply-To: <d2418c64-f1c7-810d-b80e-91155e0aaffd@lechnology.com>
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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 with the
> >>> + * Counter Signals, Synapses, and Counts respectively.
> >>> + */
> >>> +int counter_register(struct counter_device *const counter)
> >>> +{
> >>> + struct device *const dev = &counter->dev;
> >>> + int err;
> >>> +
> >>> + /* Acquire unique ID */
> >>> + counter->id = ida_simple_get(&counter_ida, 0, 0, GFP_KERNEL);
> >>> + if (counter->id < 0)
> >>> + return counter->id;
> >>> +
> >>> + /* Configure device structure for Counter */
> >>> + dev->type = &counter_device_type;
> >>> + dev->bus = &counter_bus_type;
> >>> + if (counter->parent) {
> >>> + dev->parent = counter->parent;
> >>> + dev->of_node = 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 = counter_sysfs_add(counter);
> >>> + if (err)
> >>> + goto err_free_id;
> >>> +
> >>> + /* Add device to system */
> >>> + err = 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.
> >
> > 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().
>
> add_device() also releases this reference by calling put_device() in all
> return paths. The reference count is initialized to 1 in device_initialize().
>
> 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.
> >
> > 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().
> >
> > 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().
> >
> > Is my logic correct here?
>
> 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.
William Breathitt Gray
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next prev parent reply other threads:[~2020-10-23 13:13 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-27 2:18 [PATCH v5 0/5] Introduce the Counter character device interface William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 1/5] counter: Internalize sysfs interface code William Breathitt Gray
2020-10-13 2:15 ` David Lechner
2020-10-18 14:49 ` William Breathitt Gray
2020-10-20 15:38 ` David Lechner
2020-10-23 13:12 ` William Breathitt Gray [this message]
2020-10-15 1:38 ` David Lechner
2020-10-18 17:00 ` William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 2/5] docs: counter: Update to reflect sysfs internalization William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 3/5] counter: Add character device interface William Breathitt Gray
2020-10-14 1:40 ` David Lechner
2020-10-18 16:49 ` William Breathitt Gray
2020-10-20 15:53 ` David Lechner
2020-10-25 12:55 ` William Breathitt Gray
2020-10-25 16:36 ` David Lechner
2020-10-14 17:43 ` David Lechner
2020-10-14 19:05 ` William Breathitt Gray
2020-10-14 22:32 ` David Lechner
2020-10-14 22:40 ` David Lechner
2020-10-18 16:58 ` William Breathitt Gray
2020-10-20 16:06 ` David Lechner
2020-10-25 13:18 ` William Breathitt Gray
2020-10-25 16:34 ` David Lechner
2020-10-25 17:53 ` William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 4/5] docs: counter: Document " William Breathitt Gray
2020-10-08 8:09 ` Pavel Machek
2020-10-08 12:28 ` William Breathitt Gray
2020-10-12 17:04 ` David Lechner
2020-10-13 18:58 ` William Breathitt Gray
2020-10-13 19:08 ` David Lechner
2020-10-13 19:27 ` William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 5/5] counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8 William Breathitt Gray
2020-10-14 0:13 ` David Lechner
2020-10-18 14:50 ` William Breathitt Gray
2020-10-13 0:35 ` [PATCH v5 0/5] Introduce the Counter character device interface David Lechner
2020-10-18 14:14 ` William Breathitt Gray
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