Re: [PATCH] driver core: better reference counting for klists

[Greg KH <greg@kroah.com>]

On Thu, Feb 23, 2006 at 10:38:13AM -0500, Alan Stern wrote:
> On Wed, 22 Feb 2006, Greg KH wrote:
> 
> > > Index: usb-2.6/drivers/base/dd.c
> > > ===================================================================
> > > --- usb-2.6.orig/drivers/base/dd.c
> > > +++ usb-2.6/drivers/base/dd.c
> > > @@ -72,6 +72,8 @@ int driver_probe_device(struct device_dr
> > >  {
> > >     int ret = 0;
> > >     
> > > +   if (!device_is_registered(dev))
> > > +           return -ENODEV;
> > >     if (drv->bus->match && !drv->bus->match(dev, drv))
> > >             goto Done;
> > >     
> > > Index: usb-2.6/drivers/base/bus.c
> > > ===================================================================
> > > --- usb-2.6.orig/drivers/base/bus.c
> > > +++ usb-2.6/drivers/base/bus.c
> > > @@ -367,6 +367,7 @@ int bus_add_device(struct device * dev)
> > >  
> > >  	if (bus) {
> > >  		pr_debug("bus %s: add device %s\n", bus->name, dev->bus_id);
> > > +		dev->is_registered = 1;
> > >  		device_attach(dev);
> > >  		klist_add_tail(&dev->knode_bus, &bus->klist_devices);
> > >  		error = device_add_attrs(bus, dev);
> > > @@ -393,7 +394,8 @@ void bus_remove_device(struct device * d
> > >  		sysfs_remove_link(&dev->kobj, "bus");
> > >  		sysfs_remove_link(&dev->bus->devices.kobj, dev->bus_id);
> > >  		device_remove_attrs(dev->bus, dev);
> > > -		klist_remove(&dev->knode_bus);
> > > +		klist_del(&dev->knode_bus);
> > > +		dev->is_registered = 0;
> > 
> > Don't we have a race between these two lines?  How is that protected?
> 
> Are you referring to the two lines that set dev->is_registered?  There is 
> no direct protection.  However, one line is in bus_add_device() and the 
> other is in bus_remove_device(); I've been assuming that any code 
> responsible for adding and removing devices is serialized.  That is, it 
> won't ever try to remove a device before that device has been completely 
> added.

Yes, that's probably safe to say.

> If that assumption isn't true, there are undoubtedly many other similar 
> problems throughout the driver core.  Like the calls to sysfs_create_link 
> in bus_add_device and sysfs_remove_link in bus_remove_device.
> 
> Or maybe you're referring to the device_is_registered() test in 
> driver_probe_device().  That's synchronized with the call to 
> device_release_driver() in bus_remove_device(), just below the portion you 
> quoted, because both routines hold dev->sem.  So even if the probe routine 
> fails to see that the device has been unregistered, we are guaranteed that 
> device_release_driver will unbind the device.
> 
> If you're referring to two other lines, which lines are they?

The last 2 ones above, doing a klist_del() and then after that setting
is_registered to 0.

thanks,

greg k-h