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Message-ID: <ZeJcTl0WuegYHe2h@boqun-archlinux>
Date: Fri, 1 Mar 2024 14:53:02 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: mathys35.gasnier@...il.com
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...il.com>,
Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>,
Andreas Hindborg <a.hindborg@...sung.com>,
Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org,
Martin Rodriguez Reboredo <yakoyoku@...il.com>
Subject: Re: [PATCH v5] rust: locks: Add `get_mut` method to `Lock`
On Fri, Mar 01, 2024 at 06:33:23PM +0100, Mathys-Gasnier via B4 Relay wrote:
> From: Mathys-Gasnier <mathys35.gasnier@...il.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 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.
>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Mathys-Gasnier <mathys35.gasnier@...il.com>
> ---
> Changes in v5:
> - Adding example
> - Link to v4: https://lore.kernel.org/r/20240226-rust-locks-get-mut-v4-1-24abf57707a8@gmail.com
>
> Changes in v4:
> - Improved documentation
> - Link to v3: https://lore.kernel.org/r/20240222-rust-locks-get-mut-v3-1-d38a6f4bde3d@gmail.com
>
> Changes in v3:
> - Changing the function to take a `Pin<&mut self>` instead of a `&mut self`
> - Removed reviewed-by's since big changes were made. Please take another
> look.
> - Link to v2: https://lore.kernel.org/r/20240212-rust-locks-get-mut-v2-1-5ccd34c2b70b@gmail.com
>
> Changes in v2:
> - Improved doc comment.
> - Link to v1: https://lore.kernel.org/r/20240209-rust-locks-get-mut-v1-1-ce351fc3de47@gmail.com
> ---
> rust/kernel/sync/lock.rs | 38 +++++++++++++++++++++++++++++++++++++-
> 1 file changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f12a684bc957..345ca7be9d9f 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -7,7 +7,11 @@
>
> use super::LockClassKey;
> use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use core::{
> + cell::UnsafeCell,
> + marker::{PhantomData, PhantomPinned},
> + pin::Pin,
> +};
> use macros::pin_data;
>
> pub mod mutex;
> @@ -121,6 +125,38 @@ 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.
> + ///
> + /// Having a mutable reference to the lock guarantees that no other threads have access to the
> + /// lock. And because `data` is not structurally pinned, it is safe to get a mutable reference
> + /// to the lock content.
> + ///
> + /// # Example
> + ///
Thanks! But please see below:
> + /// Using `get_mut` with a mutex.
> + ///
> + /// ```
The example looks good, however, I was thinking about something like:
/// ```
/// use kernel::sync::{new_mutex, Mutex};
///
/// let mut m = Box::pin_init(new_mutex!(None))?;
///
/// assert_eq!(*(m.lock()), None);
///
/// Mutex::get_mut(m.as_mut()).replace(42i32);
///
/// assert_eq!(*(m.lock()), Some(42));
///
/// # Ok::<(), Error>(())
/// ```
because, this will also run something instead of just compiling a
function.
> + /// use kernel::sync::Mutex;
> + ///
> + /// struct Example {
> + /// a: u32,
> + /// b: u32,
> + /// }
> + ///
> + /// fn example(m: Pin<&mut Mutex<Example>>) {
> + /// // Calling from Mutex to avoid conflict with Pin::get_mut().
> + /// let mut data = Mutex::get_mut(m);
The other thing I notice when I try to make the above example work is:
`Pin` also has a `get_mut`[1] function, so seems we have to use
`Mutex::get_mut` to invoke the correct function, I personally want the
following just works:
m.as_mut().get_mut().replace(42i32);
and looks to me the simplest way is to change the function's name (for
example `get_data_mut`), and we can do:
m.as_mut().get_data_mut().replace(42i32);
Thoughts?
Regards,
Boqun
[1]: https://doc.rust-lang.org/core/pin/struct.Pin.html#method.get_mut
> + /// data.a += 10;
> + /// data.b += 20;
> + /// }
> + /// ```
> + pub fn get_mut(self: Pin<&mut Self>) -> &mut T {
> + // SAFETY: The lock will only be used to get a reference to the data, therefore self won't
> + // get moved.
> + let lock = unsafe { self.get_unchecked_mut() };
> + lock.data.get_mut()
> + }
> }
>
> /// A lock guard.
>
> ---
> base-commit: 711cbfc717650532624ca9f56fbaf191bed56e67
> change-id: 20240118-rust-locks-get-mut-c42072101d7a
>
> Best regards,
> --
> Mathys-Gasnier <mathys35.gasnier@...il.com>
>
>
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