On Wed, 18 Apr 2007, Dmitry Torokhov wrote:
> I am still do not understand why this is needed. Would it not be
> simplier just to use a reference to struct device instead of embedding
> it in a larger structure if their lifetimes are different and one does
> not have a subsystem that takes care of releasing logic.
>
>
> Pretty much drivers have 2 options:
>
> struct my_device {
> void *private_data;
> struct device dev;
> };
Actually people use dev_[gs]et_drvdata() instead of a separate
private_data pointer. That way there's no need for the my_device
container.
> In this case ->release must live in a subsystem code; individual
> drivers kfree(my_dev->private) and do any additional cleanup after
> calling device_unregister(&my_dev->dev);
That doesn't sound right. Generally the call to device_unregister() and
the release method live in the same module. Maybe you meant to say
individual drivers kfree(my_dev->private_data) and do any additional
cleanup in their remove() routine.
This approach seems dangerous. Suppose there's mutex embedded in
my_dev->private_data, and suppose some other thread is blocked waiting on
that mutex when remove() is called. That other thread will then oops when
my_dev->private_data is deallocated.
> Second option:
>
> struct my_device {
> type member1;
> type member2;
>
> struct device *dev;
> };
>
> dev is coming from _device_create(). Driver core takes care of
> releasing dev structure; driver does cleanup of my_device.
Lots of drivers create devices dynamically without using device_create().
More to the point, how does the driver clean up my_device? It probably
has a reference count somewhere in my_device, especially if my_device is
shared with other threads or other drivers. We then face exactly the same
problem: What happens if the driver's module is unloaded before all the
references to my_device are gone?
> With current sysfs orphaning attributes upon removal request there is
> no issue of accessing driver-private data through references obtained
> via ether embedded or referenced dev structure so everything is fine.
Not so. There are other pathways besides sysfs which can cause a driver
to access its data structures.
Alan Stern
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