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Re: [Xen-devel] [PATCH 0/5] Get rid of get_driver() and put_driver()

On Tue, Jan 24, 2012 at 01:33:37PM -0500, Alan Stern wrote:
> Greg:
> This patch series removes the get_driver() and put_driver() routines
> from the kernel.
> Those routines don't do anything useful.  Their comments say that they
> increment and decrement the driver's reference count, just like
> get_device()/put_device() and a lot of other utility routines.  But a
> struct driver is _not_ like a struct device!  It resembles a piece of
> code more than a piece of data -- it acts as an encapsulation of a
> driver.  Incrementing its refcount doesn't have much meaning because a
> driver's lifetime isn't determined by the structure's refcount; it's
> determined by when the driver's module gets unloaded.
> What really matters for a driver is whether or not it is registered.  
> Drivers expect, for example, that none of their methods will be called
> after driver_unregister() returns.  It doesn't matter if some other
> thread still holds a reference to the driver structure; that reference
> mustn't be used for accessing the driver code after unregistration.  
> get_driver() does not do any checking for this.
> People may have been misled by the kerneldoc into thinking that the
> references obtained by get_driver() do somehow pin the driver structure
> in memory.  This simply isn't true; all it pins is the associated
> private structure.  Code that needs to pin a driver must do it some
> other way (probably by calling try_module_get()).
> In short, these routines don't do anything useful and they can actively 
> mislead people.  Removing them won't introduce any bugs that aren't 
> already present.  There is no reason to keep them.

Very nice work, all now applied, thanks for doing this.

greg k-h

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