Messages in this thread | | | Date | Mon, 29 Nov 2010 21:11:13 +0100 (CET) | From | Guennadi Liakhovetski <> | Subject | Re: [PATCH/RFC] core: add a function to safely try to get device driver owner |
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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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