linux-kernel - Re: [PATCH v2] rust: locks: Add `get

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Date: Thu, 15 Feb 2024 20:55:11 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Wedson Almeida Filho <wedsonaf@...il.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: mathys35.gasnier@...il.com, Miguel Ojeda <ojeda@...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@...sung.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, Martin Rodriguez Reboredo <yakoyoku@...il.com>
Subject: Re: [PATCH v2] rust: locks: Add `get_mut` method to `Lock`

On 15.02.24 17:50, Wedson Almeida Filho wrote:
> On Mon, 12 Feb 2024 at 11:22, Alice Ryhl <aliceryhl@...gle.com> wrote:
>>
>> On Mon, Feb 12, 2024 at 3:13 PM Mathys-Gasnier via B4 Relay
>> <devnull+mathys35.gasnier.gmail.com@...nel.org> wrote:
>>> +    /// Gets the data contained in the lock
>>> +    /// Having a mutable reference to the lock guarantees that no other threads have access to the lock.
>>> +    /// Making it safe to get a mutable reference to the lock content.
>>> +    pub fn get_mut(&mut self) -> &mut T {
>>> +        self.data.get_mut()
>>> +    }
>>
>> It's impossible to call this method. You can never have a mutable
>> reference to a Linux mutex because we pin our locks. At most, you can
>> have a Pin<&mut Self>.
> 
> Perhaps you meant to say that it's impossible to call this method
> without unsafe blocks? From a `Pin<&mut T>`, we can call
> `get_unchecked_mut` to get an `&mut T`.

That is the wrong way to design this, since it forces an extra
`unsafe` call. Instead this function's receiver type should be
`self: Pin<&mut Self>`.

-- 
Cheers,
Benno