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Date: Thu, 15 Feb 2024 20:55:11 +0000
To: Wedson Almeida Filho <wedsonaf@gmail.com>, Alice Ryhl <aliceryhl@google.com>
From: Benno Lossin <benno.lossin@proton.me>
Cc: mathys35.gasnier@gmail.com, Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, Andreas Hindborg <a.hindborg@samsung.com>, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Subject: Re: [PATCH v2] rust: locks: Add `get_mut` method to `Lock`
Message-ID: <ff06ddd4-808a-44fd-9230-f60aa43ec1de@proton.me>
In-Reply-To: <CANeycqqwGE_eyFSDWkmHCBOMZ6+Ty-XK_hKixMtOYqc9_46j2A@mail.gmail.com>
References: <20240212-rust-locks-get-mut-v2-1-5ccd34c2b70b@gmail.com> <CAH5fLghTKpLs=Wb03cjx0LRQbZgmXRoMNy1cyheb7cQRL0mjsw@mail.gmail.com> <CANeycqqwGE_eyFSDWkmHCBOMZ6+Ty-XK_hKixMtOYqc9_46j2A@mail.gmail.com>
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On 15.02.24 17:50, Wedson Almeida Filho wrote:
> On Mon, 12 Feb 2024 at 11:22, Alice Ryhl <aliceryhl@google.com> wrote:
>>
>> On Mon, Feb 12, 2024 at 3:13=E2=80=AFPM Mathys-Gasnier via B4 Relay
>> <devnull+mathys35.gasnier.gmail.com@kernel.org> wrote:
>>> +    /// Gets the data contained in the lock
>>> +    /// Having a mutable reference to the lock guarantees that no othe=
r 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>.
>=20
> 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>`.

--=20
Cheers,
Benno