[PATCH v4] rust: locks: Add `get_mut` method to `Lock`

[Mathys-Gasnier via B4 Relay <devnull+mathys35.gasnier.gmail.com@kernel.org>]

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.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Mathys-Gasnier <mathys35.gasnier@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 | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index f12a684bc957..a563991bf851 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 +125,17 @@ 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.
+    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@gmail.com>