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

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Message-ID: <aEAUlvPbX9vFWhdI@google.com>
Date: Wed, 4 Jun 2025 09:40:38 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Christian Schrefl <chrisi.schrefl@...il.com>
Cc: Benno Lossin <lossin@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Danilo Krummrich <dakr@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Arnd Bergmann <arnd@...db.de>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Lee Jones <lee@...nel.org>, 
	Daniel Almeida <daniel.almeida@...labora.com>, 
	"Gerald Wisböck" <gerald.wisboeck@...ther.ink>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] rust: miscdevice: add additional data to MiscDeviceRegistration

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