Re: [PATCH/RFC] core: add a function to safely try to get device driver owner

Greg KH
2010-11-29T19:56:58Z

On Mon, Nov 29, 2010 at 08:43:28PM +0100, Guennadi Liakhovetski wrote:
> When two drivers interoperate without an explicit dependency, it is often
> required to prevent one of them from being unloaded safely by dereferencing
> dev->driver->owner. This patch provides a generic function to do this in a
> race-free way.
>
> Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> ---
>
> Not run-time tested in this form, but this is just a generalisation of the
> code in drivers/media/video/sh_mobile_ceu_camera.c::sh_mobile_ceu_probe().
> If the idea is accepted in principle, I will replace that specific
> implementation with a call to this function, test... But I am not sure, if
> I'd be able to test it for races. If such testing is required on SMP, I'd
> have to write some test-case for it. Thoughts?
>
> drivers/base/dd.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/device.h | 1 +
> 2 files changed, 64 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index da57ee9..44c6672 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -17,10 +17,12 @@
> * This file is released under the GPLv2
> */
>
> +#include <linux/completion.h>
> #include <linux/device.h>
> #include <linux/delay.h>
> #include <linux/module.h>
> #include <linux/kthread.h>
> +#include <linux/notifier.h>
> #include <linux/wait.h>
> #include <linux/async.h>
> #include <linux/pm_runtime.h>
> @@ -422,3 +424,64 @@ void dev_set_drvdata(struct device *dev, void *data)
> dev->p->driver_data = data;
> }
> EXPORT_SYMBOL(dev_set_drvdata);
> +
> +struct bus_wait {
> + struct notifier_block notifier;
> + struct completion completion;
> + struct device *dev;
> +};
> +
> +static int bus_notify(struct notifier_block *nb,
> + unsigned long action, void *data)
> +{
> + struct device *dev = data;
> + struct bus_wait *wait = container_of(nb, struct bus_wait, notifier);
> +
> + if (wait->dev != dev)
> + return NOTIFY_DONE;
> +
> + switch (action) {
> + case BUS_NOTIFY_UNBOUND_DRIVER:
> + /* Protect from module unloading */
> + wait_for_completion(&wait->completion);
> + return NOTIFY_OK;
> + }
> + return NOTIFY_DONE;
> +}
> +
> +int device_try_get_driver(struct device *dev)

Please create some kerneldoc information for this new function so that
people know how to use it and what it is for.

Also, do you want to provide a device_put_driver() function as well to
decrement the owner count once the person who grabed the driver is done
with it?

> +{
> + struct bus_wait wait = {
> + .completion = COMPLETION_INITIALIZER_ONSTACK(wait.completion),
> + .dev = dev,
> + .notifier.notifier_call = bus_notify,
> + };
> + struct bus_type *bus;
> + int ret;
> +
> + if (!dev || !dev->bus)
> + return 0;
> +
> + bus = dev->bus;
> +
> + if (bus_register_notifier(bus, &wait.notifier) < 0)
> + return 0;
> +
> + /*
> + * From this point the driver module will not unload, until we complete
> + * the completion. In the worst case it is hanging in device release on
> + * our completion. So, _now_ dereferencing the "owner" is safe.
> + */
> + if (dev->driver && dev->driver->owner)
> + ret = try_module_get(dev->driver->owner);
> + else
> + /* Either no driver, or too late, or probing failed */
> + ret = 0;
> +
> + /* Let notifier complete, if it has been blocked */
> + complete(&wait.completion);
> + bus_unregister_notifier(bus, &wait.notifier);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL(device_try_get_driver);

EXPORT_SYMBOL_GPL() please.

thanks,

greg k-h