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Date: Fri, 9 Feb 2024 20:44:11 +0100
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Subject: Re: [PATCH] rust: locks: Add `get_mut` method to `Lock`
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To: mathys35.gasnier@gmail.com
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
 Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>,
 Wedson Almeida Filho <wedsonaf@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>,
 Benno Lossin <benno.lossin@proton.me>,
 Andreas Hindborg <a.hindborg@samsung.com>, Alice Ryhl <aliceryhl@google.com>
References: <20240209-rust-locks-get-mut-v1-1-ce351fc3de47@gmail.com>
From: Alice Ryhl <alice@ryhl.io>
In-Reply-To: <20240209-rust-locks-get-mut-v1-1-ce351fc3de47@gmail.com>
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On 2/9/24 17:22, Mathys-Gasnier via B4 Relay wrote:
> From: Mathys-Gasnier <mathys35.gasnier@gmail.com>
> 
> Having a mutable reference guarantees that no other threads have
> access to the lock, so we can take advantage of that to grant callers
> access to the protected data without the the cost of acquiring and
> releasing the locks. Since the lifetime of the data is tied to the
> mutable reference, the borrow checker guarantees that the usage is safe.
> 
> Signed-off-by: Mathys-Gasnier <mathys35.gasnier@gmail.com>
> ---
>   rust/kernel/sync/lock.rs | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f12a684bc957..7711c6313e14 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -121,6 +121,11 @@ pub fn lock(&self) -> Guard<'_, T, B> {
>           // SAFETY: The lock was just acquired.
>           unsafe { Guard::new(self, state) }
>       }
> +
> +    /// Gets the data contained in the lock
> +    pub fn get_mut(&mut self) -> &mut T {
> +        self.data.get_mut()
> +    }

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>.

Alice