On Fri, 4 Dec 2009, Oliver Neukum wrote:
> > > 1. am I supposed to get a reference just so that I can use dev_err?
> >
> > No, you should already have a reference on the device when doing the
> > call, right?
>
> No, why? Consider this:
>
> int write(...)
> {
> ...
> mutex_lock(&instance->lock);
> if (instance->disconnected) {
> dev_dbg(instance->dev,"writing to disconnected device");
> rv = -ENODEV;
> } else {
> res = usb_submit_urb(...);
> rv = res < 0 ? -EIO : count;
> }
> mutex_unlock(&instance->lock);
> return rv;
> }
>
> void disconnect(...)
> {
> ...
> mutex_lock(&instance->lock);
> instance->disconnected = 1;
> usb_kill_urb(...);
> usb_kill_urb(...);
> mutex_unlock(&instance->lock);
> }
>
> This would be perfectly valid code without any references taken save
> for the pesky dev_dbg()
Whoever calls write() must possess a valid reference. Otherwise
instance might already be deallocated when write() starts, causing an
oops well before the call to dev_dbg().
Typically the driver would take a reference during open() and drop it
during close().
Alan Stern
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