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From: Wedson Almeida Filho <wedsonaf@gmail.com>
Date: Thu, 15 Feb 2024 13:50:58 -0300
Message-ID: <CANeycqqwGE_eyFSDWkmHCBOMZ6+Ty-XK_hKixMtOYqc9_46j2A@mail.gmail.com>
Subject: Re: [PATCH v2] rust: locks: Add `get_mut` method to `Lock`
To: Alice Ryhl <aliceryhl@google.com>
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>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Martin Rodriguez Reboredo <yakoyoku@gmail.com>
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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>.

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`.

This is addressing issue 924 opened by Bj=C3=B6rn some time back. The idea
here is that if there's a path where avoiding the lock/unlock calls
(which are expensive because of the memory barriers) is performance
critical, we can do it as long as we use an unsafe block.