Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this_device

[Boqun Feng <boqun.feng@gmail.com>]

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@kernel.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()

No, inc_ref() is not called implicitly in Device::as_ref().

The thing that might "keep" the original `miscdevice::Device` alive is
the lifetime, since the returned `device::Device` reference has the
same life at the input parameter `miscdevice::Device` reference (i.e.
`&self`), so the returned reference cannot outlive `&self`. That means
if compilers find `&self` go out of scope while the returned reference
be still alive, it will report an error.

Regards,
Boqun

> 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]