linux-kernel - Re: [PATCH v4 12/13] gpiolib: add new ioctl() for monitoring changes in line info

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Date:   Tue, 9 Jun 2020 08:23:46 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v4 12/13] gpiolib: add new ioctl() for monitoring changes
 in line info

On Tue, Dec 24, 2019 at 01:07:08PM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> 

[snip!]

> +static int lineinfo_changed_notify(struct notifier_block *nb,
> +				   unsigned long action, void *data)
> +{
> +	struct gpio_chardev_data *priv = to_gpio_chardev_data(nb);
> +	struct gpioline_info_changed chg;
> +	struct gpio_desc *desc = data;
> +	int ret;
> +
> +	if (!test_bit(desc_to_gpio(desc), priv->watched_lines))
> +		return NOTIFY_DONE;
> +
> +	memset(&chg, 0, sizeof(chg));
> +	chg.info.line_offset = gpio_chip_hwgpio(desc);
> +	chg.event_type = action;
> +	chg.timestamp = ktime_get_real_ns();
> +	gpio_desc_to_lineinfo(desc, &chg.info);
> +

Is this call legal?  It can sleep - in fact you recently changed that
very function to move a mutex call outside of a spinlock protected section.
Here it is being called within an RCU lock, as lineinfo_changed_notify
is at the end of an atomic_notifier_call_chain.

I was looking at adding a chg.seqno here and considering what level of
locking would be appropriate for the source counter when I noticed that
call. Hopefully I'm missing something.

Cheers,
Kent.