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`.
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/rust-for-linux/20230412121431.41627-1-wedsonaf@gmail.com/
[ Removed the irqsave bits as discussed. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/sync/lock.rs

@@ -6,7 +6,7 @@
 //! spinlocks, raw spinlocks) to be provided with minimal effort.
 
 use super::LockClassKey;
-use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque};
+use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
 use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
 use macros::pin_data;
 
@@ -22,6 +22,8 @@ pub mod spinlock;
 ///
 /// - Implementers must ensure that only one thread/CPU may access the protected data once the lock
 /// is owned, that is, between calls to `lock` and `unlock`.
+/// - Implementers must also ensure that `relock` uses the same locking method as the original
+/// lock operation.
 pub unsafe trait Backend {
     /// The state required by the lock.
     type State;
@@ -55,6 +57,17 @@ pub unsafe trait Backend {
     ///
     /// It must only be called by the current owner of the lock.
     unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState);
+
+    /// Reacquires the lock, making the caller its owner.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that `guard_state` comes from a previous call to [`Backend::lock`] (or
+    /// variant) that has been unlocked with [`Backend::unlock`] and will be relocked now.
+    unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
+        // SAFETY: The safety requirements ensure that the lock is initialised.
+        *guard_state = unsafe { Self::lock(ptr) };
+    }
 }
 
 /// A mutual exclusion primitive.
@@ -126,6 +139,20 @@ 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) };
+
+        // SAFETY: The lock was just unlocked above and is being relocked now.
+        let _relock =
+            ScopeGuard::new(|| unsafe { B::relock(self.lock.state.get(), &mut self.state) });
+
+        cb();
+    }
+}
+
 impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
     type Target = T;
 

rust/kernel/sync/lock/spinlock.rs

@@ -87,7 +87,8 @@ pub type SpinLock<T> = super::Lock<T, SpinLockBackend>;
 /// A kernel `spinlock_t` lock backend.
 pub struct SpinLockBackend;
 
-// SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion.
+// SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion. `relock` uses the
+// default implementation that always calls the same locking method.
 unsafe impl super::Backend for SpinLockBackend {
     type State = bindings::spinlock_t;
     type GuardState = ();