linux-kernel - Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this

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Message-ID: <20241206074959.GL8882@google.com>
Date: Fri, 6 Dec 2024 07:49:59 +0000
From: Lee Jones <lee@...nel.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, arnd@...db.de, ojeda@...nel.org,
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net,
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out
 miscdevice::this_device

On Fri, 06 Dec 2024, Greg KH wrote:

> On Fri, Dec 06, 2024 at 07:33:09AM +0000, Lee Jones wrote:
> > On Fri, 06 Dec 2024, Lee Jones wrote:
> > 
> > > On Fri, 06 Dec 2024, Greg KH wrote:
> > > 
> > > > On Thu, Dec 05, 2024 at 04:25:18PM +0000, Lee Jones wrote:
> > > > > There are situations where a pointer to a `struct device` will become
> > > > > necessary (e.g. for calling into dev_*() functions).  This accessor
> > > > > allows callers to pull this out from the `struct miscdevice`.
> > > > > 
> > > > > Signed-off-by: Lee Jones <lee@...nel.org>
> > > > > ---
> > > > >  rust/kernel/miscdevice.rs | 9 +++++++++
> > > > >  1 file changed, 9 insertions(+)
> > > > > 
> > > > > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > > > > index 7e2a79b3ae26..55340f316006 100644
> > > > > --- a/rust/kernel/miscdevice.rs
> > > > > +++ b/rust/kernel/miscdevice.rs
> > > > > @@ -10,11 +10,13 @@
> > > > >  
> > > > >  use crate::{
> > > > >      bindings,
> > > > > +    device::Device,
> > > > >      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> > > > >      prelude::*,
> > > > >      str::CStr,
> > > > >      types::{ForeignOwnable, Opaque},
> > > > >  };
> > > > > +
> > > > >  use core::{
> > > > >      ffi::{c_int, c_long, c_uint, c_ulong},
> > > > >      marker::PhantomData,
> > > > > @@ -84,6 +86,13 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
> > > > >      pub fn as_raw(&self) -> *mut bindings::miscdevice {
> > > > >          self.inner.get()
> > > > >      }
> > > > > +
> > > > > +    /// Returns a pointer to the current Device
> > > > > +    pub fn device(&self) -> &Device {
> > > > > +        // SAFETY: This is only accessible after a successful register() which always
> > > > > +        // initialises this_device with a valid device.
> > > > > +        unsafe { Device::as_ref((*self.as_raw()).this_device) }
> > > > 
> > > > A "raw" pointer that you can do something with without incrementing the
> > > > reference count of it?  Oh wait, no, it's the rust device structure.
> > > > If so, why isn't this calling the get_device() interface instead?  That
> > > > way it's properly incremented and decremented when it "leaves the scope"
> > > > right?
> > > > 
> > > > Or am I missing something here as to why that wouldn't work and this is
> > > > the only way to get access to the 'struct device' of this miscdevice?
> > > 
> > > Fair point.  I'll speak to Alice.
> > 
> > Alice isn't available yet, so I may be talking out of turn at this
> > point, but I just found this is the Device documentation:
> > 
> >   /// A `Device` instance represents a valid `struct device` created by the C portion of the kernel.
> >   ///
> >   /// Instances of this type are always reference-counted, that is, a call to `get_device` ensures
> >   /// that the allocation remains valid at least until the matching call to `put_device`.
> > 
> > And:
> > 
> >   // SAFETY: Instances of `Device` are always reference-counted.
> > 
> > Ready for some analysis from this beginner?
> > 
> > Since this impl for Device is AlwaysRefCounted, when any references are
> > taken i.e. in the Device::as_ref line above, inc_ref() is implicitly
> > called to increase the refcount.  The same will be true of dec_ref()
> > once it goes out of scope.
> > 
> >   // SAFETY: Instances of `Device` are always reference-counted.
> >   unsafe impl crate::types::AlwaysRefCounted for Device {
> >       fn inc_ref(&self) {
> >           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
> >           unsafe { bindings::get_device(self.as_raw()) };
> >       }
> > 
> >       unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> >           // SAFETY: The safety requirements guarantee that the refcount is non-zero.
> >           unsafe { bindings::put_device(obj.cast().as_ptr()) }
> >   }
> 
> Ick, really?  So as_ref() implicitly calles inc_ref() and dec_ref()?
> Ah, ok, in digging into AlwaysRefCounted I now see that seems to be
> true.
> 
> So great, this is a reference counted object, so what's preventing it
> from now being used in dev_info()?

We're having this conversation in stereo at this point.

TL;DR, we can't call MiscDeviceRegistration::device() after .init YET.

The longer version of this can be seen in the cover-letter thread.

-- 
Lee Jones [李琼斯]