rust: lock: add trylock method support for lock backend

Add a non-blocking trylock method to lock backend interface, mutex and
spinlock implementations. It includes a C helper for spin_trylock.

Rust Binder will use this method together with the new shrinker
abstractions to avoid deadlocks in the memory shrinker.

Link: https://lore.kernel.org/all/20240912-shrinker-v1-1-18b7f1253553@google.com
Signed-off-by: Filipe Xavier <felipe_life@live.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/BL0PR02MB4914579914884B5D7473B3D6E96A2@BL0PR02MB4914.namprd02.prod.outlook.com
[ Slightly reworded. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

3 files changed, 38 insertions, 0 deletions

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 07fcf2d8efc6..90cc5416529b 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -58,6 +58,13 @@ pub unsafe trait Backend {
     #[must_use]
     unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState;
 
+    /// Tries to acquire the lock.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that [`Backend::init`] has been previously called.
+    unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState>;
+
     /// Releases the lock, giving up its ownership.
     ///
     /// # Safety
@@ -128,6 +135,15 @@ impl<T: ?Sized, B: Backend> Lock<T, B> {
         // SAFETY: The lock was just acquired.
         unsafe { Guard::new(self, state) }
     }
+
+    /// Tries to acquire the lock.
+    ///
+    /// Returns a guard that can be used to access the data protected by the lock if successful.
+    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)) }
+    }
 }
 
 /// A lock guard.
diff --git a/rust/kernel/sync/lock/mutex.rs b/rust/kernel/sync/lock/mutex.rs
index 30632070ee67..c4f3b6cbfe48 100644
--- a/rust/kernel/sync/lock/mutex.rs
+++ b/rust/kernel/sync/lock/mutex.rs
@@ -115,4 +115,15 @@ unsafe impl super::Backend for MutexBackend {
         // caller is the owner of the mutex.
         unsafe { bindings::mutex_unlock(ptr) };
     }
+
+    unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
+        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
+        let result = unsafe { bindings::mutex_trylock(ptr) };
+
+        if result != 0 {
+            Some(())
+        } else {
+            None
+        }
+    }
 }
diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
index ea5c5bc1ce12..c900ae23db76 100644
--- a/rust/kernel/sync/lock/spinlock.rs
+++ b/rust/kernel/sync/lock/spinlock.rs
@@ -114,4 +114,15 @@ unsafe impl super::Backend for SpinLockBackend {
         // caller is the owner of the spinlock.
         unsafe { bindings::spin_unlock(ptr) }
     }
+
+    unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
+        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
+        let result = unsafe { bindings::spin_trylock(ptr) };
+
+        if result != 0 {
+            Some(())
+        } else {
+            None
+        }
+    }
 }