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    SubjectRe: [PATCH/RFC] core: add a function to safely try to get device driver owner
    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
    http://www.open-technology.de/


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