[RFC PATCH 08/11] rust: lock: implement `IrqSaveBackend` for `SpinLock`

[Andreas Hindborg <nmi@metaspace.dk>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

This allows Rust code to use the `lock_irqsave` variant of spinlocks.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/helpers.c                    | 16 ++++++++++++
 rust/kernel/sync/lock/spinlock.rs | 41 +++++++++++++++++++++++++------
 2 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/rust/helpers.c b/rust/helpers.c
index a59341084774..7f5b95f652e1 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -203,6 +203,22 @@ struct page *rust_helper_alloc_pages(gfp_t gfp_mask, unsigned int order)
 }
 EXPORT_SYMBOL_GPL(rust_helper_alloc_pages);
 
+unsigned long rust_helper_spin_lock_irqsave(spinlock_t *lock)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(lock, flags);
+
+	return flags;
+}
+EXPORT_SYMBOL_GPL(rust_helper_spin_lock_irqsave);
+
+void rust_helper_spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
+{
+	spin_unlock_irqrestore(lock, flags);
+}
+EXPORT_SYMBOL_GPL(rust_helper_spin_unlock_irqrestore);
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
index e39142a8148c..50b8775bb49d 100644
--- a/rust/kernel/sync/lock/spinlock.rs
+++ b/rust/kernel/sync/lock/spinlock.rs
@@ -64,6 +64,8 @@ macro_rules! new_spinlock {
 /// assert_eq!(e.c, 10);
 /// assert_eq!(e.d.lock().a, 20);
 /// assert_eq!(e.d.lock().b, 30);
+/// assert_eq!(e.d.lock_irqsave().a, 20);
+/// assert_eq!(e.d.lock_irqsave().b, 30);
 /// ```
 ///
 /// The following example shows how to use interior mutability to modify the contents of a struct
@@ -81,6 +83,12 @@ macro_rules! new_spinlock {
 ///     let mut guard = m.lock();
 ///     guard.a += 10;
 ///     guard.b += 20;
+///
+/// fn example2(m: &SpinLock<Example>) {
+///     let mut guard = m.lock_irqsave();
+///     guard.a += 10;
+///     guard.b += 20;
+/// }
 /// }
 /// ```
 ///
@@ -94,7 +102,7 @@ pub struct SpinLockBackend;
 // default implementation that always calls the same locking method.
 unsafe impl super::Backend for SpinLockBackend {
     type State = CachePadded<bindings::spinlock_t>;
-    type GuardState = ();
+    type GuardState = Option<core::ffi::c_ulong>;
 
     unsafe fn init(
         ptr: *mut Self::State,
@@ -110,13 +118,32 @@ unsafe impl super::Backend for SpinLockBackend {
     unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
         // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
         // memory, and that it has been initialised before.
-        unsafe { bindings::spin_lock((&mut *ptr).deref_mut()) }
+        unsafe { bindings::spin_lock((&mut *ptr).deref_mut()) };
+        None
     }
 
-    #[inline(always)]
-    unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
-        // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
-        // caller is the owner of the mutex.
-        unsafe { bindings::spin_unlock((&mut *ptr).deref_mut()) }
+    unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState) {
+        match guard_state {
+            // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that
+            // the caller is the owner of the mutex.
+            Some(flags) => unsafe {
+                bindings::spin_unlock_irqrestore((&mut *ptr).deref_mut(), *flags)
+            },
+            // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that
+            // the caller is the owner of the mutex.
+            None => unsafe { bindings::spin_unlock((&mut *ptr).deref_mut()) },
+        }
+    }
+}
+
+// SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion. We use the `irqsave`
+// variant of the C lock acquisition functions to disable interrupts and retrieve the original
+// interrupt state, and the `irqrestore` variant of the lock release functions to restore the state
+// in `unlock` -- we use the guard context to determine which method was used to acquire the lock.
+unsafe impl super::IrqSaveBackend for SpinLockBackend {
+    unsafe fn lock_irqsave(ptr: *mut Self::State) -> Self::GuardState {
+        // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
+        // memory, and that it has been initialised before.
+        Some(unsafe { bindings::spin_lock_irqsave((&mut *ptr).deref_mut()) })
     }
 }
-- 
2.40.0