Re: [PATCH RFC v3 3/3] pinctrl: add pinctrl gpio binding support

On Thu, May 24, 2012 at 11:22:13PM +0800, Stephen Warren wrote:

On 05/23/2012 11:19 PM, Dong Aisheng wrote:

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On Thu, May 24, 2012 at 12:42:19PM +0800, Stephen Warren wrote:

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On 05/23/2012 07:42 PM, Dong Aisheng wrote:

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On Thu, May 24, 2012 at 4:44 AM, Stephen Warren [off-list ref] wrote:

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On 05/23/2012 07:22 AM, Dong Aisheng wrote:

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From: Dong Aisheng <redacted>

This patch implements a standard common binding for pinctrl gpio ranges.
Each SoC can add gpio ranges through device tree by adding a gpio-maps property
under their pinctrl devices node with the format:
<&gpio $gpio_offset $pin_offset $npin>.

Then the pinctrl driver can call pinctrl_dt_add_gpio_ranges(pctldev, node)
to parse and register the gpio ranges from device tree.

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Re: your locking comments in your other email: ranges[i].gc doesn't
appear to be used anywhere else in pinctrl, so I think it's OK not to
lock the GPIO chip for any more time than between the above two blocks
of code.

So i will add lock between them like:
ranges[i].gc = of_node_to_gpiochip(np_gpio);
if (!try_module_get(ranges[i].gc->owner))
    err...

I think that module_get() needs to happen inside of_node_to_gpiochip(),
so that it executes inside any lock that function takes.

Can you please help explain a bit more?
I did not quite understand.
It looks to me of_node_to_gpiochip is only convert the gpio node to gpio chip.
Why need get the module inside this function?
For gpio_request function, it also calls try_module_get(gc) after find the gpio
chip.

The problem is this:

Thread 1: Call of_node_to_gpiochip(), returns a gpio_chip.
Thread 2: Unregisters the same gpio_chip that was returned above.
Thread 1: Accesses the now unregistered (and possibly free'd) gpio_chip
-> at best, bad data, at worst, OOPS.

Correct. We did have this issue.
Thanks for clarify.

In order to prevent this, of_node_to_gpiochip() should take measures to
prevent another thread from unregistering the gpio_chip until thread 1
has completed its step above.

The existing of_get_named_gpio_flags() is safe from this, since
gpiochip_find() acquires the GPIO lock, and all accesses to the fouond
gpio chip occur with that lock held, inside the match function. Perhaps
a similar approach could be used here.

Why it looks to me of_get_named_gpio_flags has the same issue and also not safe?
For of_node_to_gpiochip itself called in of_get_named_gpio_flags, it's safe.
But after that, i'm suspecting it has the same issue as you described above, right?

For example:
int of_get_named_gpio_flags(struct device_node *np, const char *propname,
                           int index, enum of_gpio_flags *flags)
{
...
	gc = of_node_to_gpiochip(gpiospec.np);
	if (!gc) {
		pr_debug("%s: gpio controller %s isn't registered\n",
			 np->full_name, gpiospec.np->full_name);
		ret = -ENODEV;
		goto err1;
	}

	===> the gc may be unregistered here by another thread and
	     even already have been freed, right?

	ret = gc->of_xlate(gc, &gpiospec, flags);
...
}

Maybe we need get the lock in of_node_to_gpiochip and release it by calling
of_gpio_put(..) after using?

Regards
Dong Aisheng