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

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Message-ID: <20241206073309.GG8882@google.com>
Date: Fri, 6 Dec 2024 07:33:09 +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, 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()) }
  }

-- 
Lee Jones [李琼斯]