Hi Greg, On Thu, 14 Jul 2016 15:47:35 +0900, Greg KH wrote: > On Wed, Jul 13, 2016 at 03:46:14PM +0200, Jean Delvare wrote: > > Hi all, > > > > I am currently working on the i2c-dev driver, which has just been > > converted to the non-ancestral cdev API. As I am cleaning up the > > driver, I would like to switch from static cdev initialization > > (cdev_init) to dynamic allocation (cdev_alloc.) > > > > While I was looking at other drivers to figure out how to deal with > > error cases, I found that different drivers do different things if > > cdev_add fails after cdev_alloc was called successfully. I guess some > > of them are right, others are wrong, and I'd like to know which is > > which ;-) > > > > * char/virtio_console.c, s390/char/tape_class.c, s390/char/vmur.c, > > infiniband/.../qib_file_ops.c, fuse/cuse.c, scsi/sg.c and scsi/st.g > > are calling cdev_del(cdev). > > * v4l2-core/v4l2-dev.c is calling kfree(cdev). > > * s390/char/vmlogrdr.c, uio/uio.c, tty/ty_io.c and __register_chrdev() > > are calling kobject_put(&cdev->kobj). The former explicitly says "no > > cdev_del here!" in a comment. > > > > My gut feeling is that kobject_put(&cdev->kobj) is correct, even though > > it feels strange to have to use a low-level function to clean-up after > > a higher level API call. > > > > If cdev_del(cdev) is also correct (and as I read the code it could be, > > iff calling kobj_unmap() is a no-op if kobj_map() failed - is it the > > case?), then it should be clearly documented as such, as it is > > counter-intuitive (to me, at least.) > > > > Anyone wants to comment on this? > > > > On top of this, another thing looks strange to me. cdev_add() only gets > > the parent kobj on success. However the release methods > > (cdev_default_release and cdev_dynamic_release) will put the parent > > kobj unconditionally. So it looks to me that we are over-putting the > > parent whenever cdev_add() fails. OTOH I can't see where the parent is > > set. If it is NULL then all these get and put are no-ops to start with > > and it no longer matters. But why would we be doing that? > > > > Then again, what do I know about kobj black magic... > > It's worse than you think, the kobject in a cdev is not a "real" > kobject. Well, it's kind of real, but it's only there to be used for > the kmap logic. I have a 10+ year old TODO item here that says "remove > kobj from cdev" that I really should get to one of these days. I did figure that out actually. > Anyone that touches the kobj outside of the cdev core code is probably > wrong, it's "funny" that both uio and tty do that, the maintainer of > that code must be lazy... :) Or just as confused as myself. > Let me look into what the "correct" thing to do here is, I used to know > it, need some time to refresh my memory... Did you find? > And the cdev interface has what, 4 different ways it can be used? > Another of my TODO items is to fix it all up to only use it one way, or > maybe just 2 as it does have the ability to make driver code pretty > small if you use it in unique ways... -- Jean Delvare SUSE L3 Support -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
