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

Guennadi Liakhovetski
2010-11-29T20:11:38Z

On Mon, 29 Nov 2010, Greg KH wrote:

> 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.

Ok, will do.

> 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?

Well, once you've got the driver, you put it by just calling
module_put(device->driver->owner), but you're right, I should add one for
symmetry, and there I won't guard against the race, because if someone
would be calling the put() without a successful get(), they deserve any
Oops they get anyway;)

>
> > +{
> > + 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.

Ok

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer