Linux-IIO Archive on lore.kernel.org
 help / color / Atom feed
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
Message-ID: <20201023131256.GA30908@shinobu> (raw)
In-Reply-To: <d2418c64-f1c7-810d-b80e-91155e0aaffd@lechnology.com>


[-- Attachment #1: Type: text/plain, Size: 3796 bytes --]

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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  reply index

Thread overview: 35+ 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 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
     [not found] ` <e38f6dc3a08bf2510034334262776a6ed1df8b89.1601170670.git.vilhelm.gray@gmail.com>
2020-10-13  2:15   ` [PATCH v5 1/5] counter: Internalize sysfs interface code 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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201023131256.GA30908@shinobu \
    --to=vilhelm.gray@gmail.com \
    --cc=alexandre.belloni@bootlin.com \
    --cc=alexandre.torgue@st.com \
    --cc=david@lechnology.com \
    --cc=fabrice.gasnier@st.com \
    --cc=gwendal@chromium.org \
    --cc=jic23@kernel.org \
    --cc=kamel.bouhara@bootlin.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-iio@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-stm32@st-md-mailman.stormreply.com \
    --cc=mcoquelin.stm32@gmail.com \
    --cc=patrick.havelange@essensium.com \
    --cc=syednwaris@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-IIO Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-iio/0 linux-iio/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-iio linux-iio/ https://lore.kernel.org/linux-iio \
		linux-iio@vger.kernel.org
	public-inbox-index linux-iio

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-iio


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git