linux-kernel - [PATCH 3/3] rust: sync: Add Guard::new

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Message-ID: <20241120223442.2491136-4-lyude@redhat.com>
Date: Wed, 20 Nov 2024 17:30:43 -0500
From: Lyude Paul <lyude@...hat.com>
To: rust-for-linux@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Martin Rodriguez Reboredo <yakoyoku@...il.com>,
	Valentin Obst <kernel@...entinobst.de>,
	Filipe Xavier <felipe_life@...e.com>
Subject: [PATCH 3/3] rust: sync: Add Guard::new_unchecked()

Asserting is_locked() for unsafe Guard::new() calls is quite useful for
verifying safety for callers outside the lock module. But in the lock
module, it's a bit unnecessary and a potential performance hit for safe
rust code. Mainly because it implies all safe lock acquisitions in rust
will have to run this debug assertion.

So, let's split out Guard::new() by adding a Guard::new_unchecked()
function that skips this debug assertion. Of course, we leave this function
as private and note that it is only ever intended for use in this specific
module.

Signed-off-by: Lyude Paul <lyude@...hat.com>
---
 rust/kernel/sync/lock.rs | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 0a7f2ed767423..2fd4b665ffc9a 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -166,7 +166,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
         // that `init` was called.
         let state = unsafe { B::lock(self.state.get()) };
         // SAFETY: The lock was just acquired.
-        unsafe { Guard::new(self, state) }
+        unsafe { Guard::new_unchecked(self, state) }
     }
 
     /// Tries to acquire the lock.
@@ -175,7 +175,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
     pub fn try_lock(&self) -> Option<Guard<'_, T, B>> {
         // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
         // that `init` was called.
-        unsafe { B::try_lock(self.state.get()).map(|state| Guard::new(self, state)) }
+        unsafe { B::try_lock(self.state.get()).map(|state| Guard::new_unchecked(self, state)) }
     }
 
     /// Return whether or not the lock is currently acquired.
@@ -255,6 +255,19 @@ impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
     pub unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
         debug_assert!(lock.is_locked());
 
+        // SAFETY: Our safety requirements fulfill the requirements of this function.
+        unsafe { Self::new_unchecked(lock, state) }
+    }
+
+    /// Constructs a new immutable lock guard without assertions.
+    ///
+    /// Unlike [`Guard::new`], this function does not run a debug assertion to ensure the lock has
+    /// been acquired. It should only be used in this module.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that it owns the lock.
+    unsafe fn new_unchecked(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
         Self {
             lock,
             state,
-- 
2.47.0