rust: lock: add Guard::do_unlocked

It releases the lock, executes some function provided by the caller,
then reacquires the lock. This is preparation for the implementation of
condvars, which will sleep after between unlocking and relocking.

We need an explicit `relock` method for primitives like `SpinLock` that
have an irqsave variant: we use the guard state to determine if the lock
was originally acquired with the regular `lock` function or
`lock_irqsave`.

Signed-off-by: Wedson Almeida Filho <[email protected]>

Author: wedsonaf

rust/kernel/sync/lock.rs

@@ -55,6 +55,17 @@ pub unsafe trait Backend {
     ///
     /// It must only be called by the current owner of the lock.
     unsafe fn unlock(ptr: *mut Self::State, state: &Self::GuardState);
+
+    /// Reacquires the lock, making the caller its owner.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that `state` comes from a previous call to [`Backend::lock`] that has
+    /// been unlocked with [`Backend::unlock`] and will be relocked now.
+    unsafe fn relock(ptr: *mut Self::State, state: &mut Self::GuardState) {
+        // SAFETY: The safety requirements ensure that the lock is initialised.
+        *state = unsafe { Self::lock(ptr) };
+    }
 }
 
 /// The "backend" of a lock that supports the irq-save variant.
@@ -158,6 +169,17 @@ pub struct Guard<'a, T: ?Sized, B: Backend> {
 // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
 unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
 
+impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
+    #[allow(dead_code)]
+    pub(crate) fn do_unlocked(&mut self, cb: impl FnOnce()) {
+        // SAFETY: The caller owns the lock, so it is safe to unlock it.
+        unsafe { B::unlock(self.lock.state.get(), &self.state) };
+        cb();
+        // SAFETY: The lock was just unlocked above and is being relocked now.
+        unsafe { B::relock(self.lock.state.get(), &mut self.state) };
+    }
+}
+
 impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
     type Target = T;
 

rust/kernel/sync/lock/spinlock.rs

@@ -4,6 +4,7 @@
 //!
 //! This module allows Rust code to use the kernel's `spinlock_t`.
 
+use super::IrqSaveBackend;
 use crate::bindings;
 
 /// Creates a [`SpinLock`] initialiser with the given name and a newly-created lock class.
@@ -121,10 +122,21 @@ unsafe impl super::Backend for SpinLockBackend {
             None => unsafe { bindings::spin_unlock(ptr) },
         }
     }
+
+    unsafe fn relock(ptr: *mut Self::State, state: &mut Self::GuardState) {
+        let _ = match state {
+            // SAFETY: The safety requiments of this function ensure that `ptr` has been
+            // initialised.
+            None => unsafe { Self::lock(ptr) },
+            // SAFETY: The safety requiments of this function ensure that `ptr` has been
+            // initialised.
+            Some(_) => unsafe { Self::lock_irqsave(ptr) },
+        };
+    }
 }
 
 // SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion.
-unsafe impl super::IrqSaveBackend for SpinLockBackend {
+unsafe impl 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.