Per-open file private data for the cdevs

[Pawel Jakub Dawidek]

On Mon, May 05, 2008 at 05:56:31PM +0300, Kostik Belousov wrote:
> On Mon, May 05, 2008 at 10:13:55AM +0200, Pawel Jakub Dawidek wrote:
> > Can you see if my OSD (Object-Specific-Data) KPI can be useful here?
> > I've it only in perforce for now, but I think it's what you are looking
> > for. I use it for jails and threads currently, but it is trivial to use
> > it for other objects. Take a look:
> > 
> > 	http://perforce.freebsd.org/fileViewer.cgi?FSPC=//depot/user/pjd/zfs/sys/sys/osd.h&REV=3
> > 
> > When your code is loaded/initialized you call:
> > 
> > 	static int slot;
> > 
> > 	slot = osd_file_register(mod_destroy);
> > 
> > Where mod_destroy is a function which knows how to free your private
> > data. On unload:
> > 
> > 	osd_file_deregister(slot);
> > 
> > Now, when you want to attach private data:
> > 
> > 	error = osd_file_set(fp, slot, ptr_to_your_data);
> > 
> > You can get it with:
> > 
> > 	ptr = osd_file_get(fp, slot);
> This is a nice feature, but I think that privdata is a fundamental
> internal operation of the kernel, and it would be wrong to use
> the OSD. [...]

OSD is there to be used mostly by optional modules. For example if ZFS
is not loaded there is no need to keep a pointer to its private data in
struct prison and struct thread.

> [...] In fact, the object we shall attach OSD to is the struct
> file, and we would need to export the struct file to the cdevsw operations.
> Since it both breaks current ABI and complicates the future changes to
> the struct file, I would much prefer to not engage this route.

Ok.

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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