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

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Message-ID: <311a3039-286a-49e6-b47a-75f5a173e922@gmail.com>
Date: Wed, 4 Jun 2025 11:42:39 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Alice Ryhl <aliceryhl@...gle.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 04.06.25 11:40 AM, Alice Ryhl wrote:
> 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.

Right I meant to write `Sync` sorry.

Cheers
Christian