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

Guennadi Liakhovetski
2010-11-29T19:43:34Z

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)
+{
+ 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);
diff --git a/include/linux/device.h b/include/linux/device.h
index dd48953..5932169 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -559,6 +559,7 @@ extern const char *device_get_devnode(struct device *dev,
mode_t *mode, const char **tmp);
extern void *dev_get_drvdata(const struct device *dev);
extern void dev_set_drvdata(struct device *dev, void *data);
+int device_try_get_driver(struct device *dev);

/*
* Root device objects for grouping under /sys/devices
--
1.7.2.3