Re: [PATCH 3/3] rust: miscdevice: adjust the rust_misc_device sample to use RegistrationData.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jan 24, 2025 at 10:42:53AM +0100, Alice Ryhl wrote:
> On Fri, Jan 24, 2025 at 9:06 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Fri, Jan 24, 2025 at 08:29:38AM +0100, Alice Ryhl wrote:
> > > On Thu, Jan 23, 2025 at 6:57 PM Christian Schrefl
> > > <chrisi.schrefl@gmail.com> wrote:
> > > >
> > > > Hi Alice
> > > >
> > > > On 21.01.25 4:40 PM, Alice Ryhl wrote:
> > > > > On Sun, Jan 19, 2025 at 11:11 PM Christian Schrefl
> > > > > <chrisi.schrefl@gmail.com> wrote:
> > > > >>
> > > > >> Share the mutex stored in RustMiscDevice between all instances using an Arc
> > > > >> and the RegistrationData of MiscDeviceRegistration.
> > > > >>
> > > > >> This is mostly to Demonstrate the capability to share data in this way.
> > > > >>
> > > > >> Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
> > > > >
> > > > > This change causes all open files to share the same value, instead of
> > > > > it being per-fd.
> > > >
> > > > I know, if that is unwanted I'm fine with dropping this patch,
> > > > it is mostly here to show how patch 2 can be used.
> > >
> > > Perhaps instead of changing the per-fd value, we could add a new
> > > shared value? E.g., it could have a counter for the number of open
> > > files.
> >
> > Counters don't work, sorry (think about dup() for file handles), please,
> > either make it per-file handle, or a "global" thing for the specific
> > object, don't attempt to count open/release calls, the vfs does this for
> > us already.
> 
> I mean, it's just for an example, shrug. It could also be another
> ioctl that updates the shared value.

Sure, but the number of times I've seen sample code copied into "real"
code is way too high.  Also, getting people to stop thinking they even
can count the number of open file handles is a good idea as that's an
extremely common "anti-pattern" in way too many drivers even today (i.e.
if you try to count open calls, or even restrict the number of open
calls to attempt some sort of control, you're doing it totally wrong.)

thanks,

greg k-h