On Mon, 28 June 2010 Bernie Thompson [off-list ref] wrote:

On Mon, Jun 28, 2010 at 1:33 PM, Bruno Prémont
[off-list ref] wrote:

quoted

As our device may be hot-unplugged and framebuffer cannot handle
this case by itself we need to keep track of usage count so as
to release fb_info and framebuffer memory only after the last user
has closed framebuffer.

We need to do the freeing in a scheduled work as fb_release()
is called with fb_info lock held.

The udlfb (DisplayLink) framebuffer driver just got a similar fix in
the last few days:
http://git.plugable.com/gitphp/index.php?p=udlfb&a=commit&h®0502457e54904b1a9e0d67db6719182824da7c

The need to schedule work in fb_release() because fbmem.c touches it
after release is especially unfortunate (we had to do the same).

Anyone have existing thoughts about fixing the release path more centrally?

The best option IMHO would be that framebuffer_release() would free up
memory when framebuffer is not in use anymore (or last close if FB was
in use when framebuffer_release() was called).

Something like
  framebuffer_alloc()              <-- set refcount to 1
  ...
  register_framebuffer()           <-- inc refcount
  ...
  and open() of fb device would inc refcount
  and close() would decrement it

  unregister_framebuffer()         <-- dec refcount
  framebuffer_release()            <-- dec refcount

When after decrementing refcount, refcount reaches zero
close() or framebuffer_release() would free the framebuffer, calling
fbops fb_release() callback so owner can do cleanup if it has something
to do.

That way it would be easiest for drivers of removable framebuffers,
possibly also useful for kms adding/removing framebuffers for extra
monitors.

Thanks,
Bruno