Re: [patch 2.6.31-rc7] gpiolib: allow poll() on value

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 27 Aug 2009 16:50:57 -0700
David Brownell <david-b@pacbell.net> wrote:

> From: Daniel Gl__ckner <dg@emlix.com>
> Subject: [PATCH v3] gpiolib: allow poll(2) on gpio value
> 
> Many gpio chips allow to generate interrupts when the value of a pin
> changes. This patch gives usermode application the opportunity to make
> use of this feature by calling poll(2) on the /sys/class/gpio/gpioN/value
> sysfs file. The edge to trigger can be set in the edge file in the same
> directory. Possible values are "none", "rising", "falling", and "both".
> 
> Using level triggers is not possible with current sysfs since nothing
> changes the GPIO value (and the IRQ keeps triggering).  Edge triggering
> will "just work".  Note that if there was an event between read() and
> poll(), the poll() returns immediately.
> 
> Also note that this version only supports true GPIO interrupts.  Some
> later patch might be able to synthesize this behavior by timer-driven
> polling; some systems seem to need that.
>
> ...
>
> +static struct idr pdesc_idr;

There's no locking to protect this tree.  If that isn't a bug then I'd
suggest that a comment be added here explaining why.

> +static irqreturn_t gpio_sysfs_irq(int irq, void *priv)
> +{
> +	struct work_struct	*work = priv;
> +
> +	schedule_work(work);
> +	return IRQ_HANDLED;
> +}

The only place where we explicitly cancel the pending work is in
gpio_setup_irq().  Is that sufficient?  Is there any way in which the
work callback can occur after things have been
freed/closed/deinitialised/etc?