Re: [PATCH v4 2/3] rust: miscdevice: add additional data to MiscDeviceRegistration

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Jun 02, 2025 at 11:16:33PM +0200, Christian Schrefl wrote:
> On 31.05.25 2:23 PM, Benno Lossin wrote:
> > On Fri May 30, 2025 at 10:46 PM CEST, Christian Schrefl wrote:
> >> +        unsafe { core::ptr::drop_in_place(self.data.get()) };
> >>      }
> >>  }
> >>  
> >> @@ -109,6 +135,13 @@ pub trait MiscDevice: Sized {
> >>      /// What kind of pointer should `Self` be wrapped in.
> >>      type Ptr: ForeignOwnable + Send + Sync;
> >>  
> >> +    /// The additional data carried by the [`MiscDeviceRegistration`] for this [`MiscDevice`].
> >> +    /// If no additional data is required than the unit type `()` should be used.
> >> +    ///
> >> +    /// This data can be accessed in [`MiscDevice::open()`] using
> >> +    /// [`MiscDeviceRegistration::data()`].
> >> +    type RegistrationData: Sync;
> > 
> > Why do we require `Sync` here?
> 
> Needed for `MiscDeviceRegistration` to be `Send`, see response above.

Even if `MiscDeviceRegistration` is non-Send, the registration data must
still be Sync. The f_ops->open callback can *always* be called from any
thread no matter what we do here, so the data it gives you access must
always be Sync no matter what.

Alice