Tetsuo Handa wrote:
On 2024/07/13 4:42, Dan Williams wrote:
@@ -2668,8 +2670,12 @@ static int dev_uevent(const struct kobject *kobj, struct kobj_uevent_env *env) if (dev->type && dev->type->name) add_uevent_var(env, "DEVTYPE=%s", dev->type->name);
- if (dev->driver)
add_uevent_var(env, "DRIVER=%s", dev->driver->name);
- /* Synchronize with module_remove_driver() */
- rcu_read_lock();
- driver = READ_ONCE(dev->driver);
- if (driver)
add_uevent_var(env, "DRIVER=%s", driver->name);- rcu_read_unlock();
Given that read of dev->driver is protected using RCU,
@@ -97,6 +98,9 @@ void module_remove_driver(struct device_driver *drv) if (!drv) return;
where is
dev->driver = NULL;
performed prior to
It happens in __device_release_driver() and several places in the driver probe failure path. However, the point of this patch is that the "dev->driver = NULL" event does not really matter for this sysfs attribute.
This attribute just wants to opportunistically report the driver name to userspace, but that result is ephemeral. I.e. as soon as a dev_uevent() adds a DRIVER environment variable that result could be immediately invalidated before userspace has a chance to do anything with the result.
Even with the current device_lock() solution userspace can not depend on the driver still being attached when it goes to act on the DRIVER environment variable.
- /* Synchronize with dev_uevent() */
- synchronize_rcu();
this synchronize_rcu(), in order to make sure that READ_ONCE(dev->driver) in dev_uevent() observes NULL?
No, this synchronize_rcu() is to make sure that if dev_uevent() wins the race and observes that dev->driver is not NULL that it is still safe to dereference that result because the 'struct device_driver' object is still live.
A 'struct device_driver' instance is typically static data in a kernel module that does not get freed until after driver_unregister(). Calls to driver_unregister() typically only happen at module removal time. So this synchronize_rcu() delays module removal until dev_uevent() finishes reading driver->name.