[PATCH] rust: lock: Export Guard::do_unlocked()

[Lyude Paul]

In RVKMS, I discovered a silly issue where as a result of our HrTimer for
vblank emulation and our vblank enable/disable callbacks sharing a
spinlock, it was possible to deadlock while trying to disable the vblank
timer.

The solution for this ended up being simple: keep track of when the HrTimer
could potentially acquire the shared spinlock, and simply drop the spinlock
temporarily from our vblank enable/disable callbacks when stopping the
timer. And do_unlocked() ended up being perfect for this.

Since this seems like it's useful, let's export this for use by the rest of
the world and write short documentation for it.

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
---
You can find an example usage of this here:

  https://gitlab.freedesktop.org/lyudess/linux/-/blob/rvkms-slim/drivers/gpu/drm/rvkms/crtc.rs

 rust/kernel/sync/lock.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index e82fa5be289c1..60eb98805a489 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -228,7 +228,13 @@ pub fn lock_ref(&self) -> &'a Lock<T, B> {
         self.lock
     }
 
-    pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
+    /// Unlock this [`Guard`]'s lock temporarily, execute `cb`, and then re-lock it.
+    ///
+    /// This can be useful for situations where you may need to do a temporary unlock dance to avoid
+    /// issues like circular locking dependencies.
+    ///
+    /// If the closure returns a value, it will be returned by this function.
+    pub fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
         // SAFETY: The caller owns the lock, so it is safe to unlock it.
         unsafe { B::unlock(self.lock.state.get(), &self.state) };
 

base-commit: dff64b072708ffef23c117fa1ee1ea59eb417807
-- 
2.50.0