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@redhat.com>
---
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

On Thu, Jul 24, 2025 at 01:25:00PM -0400, Lyude Paul wrote:
> 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@redhat.com>

Seems reasonable.

> ---
> 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.

I would like to change this line as the following to be consistent with
other comments in the file.

/// Releases this [`Guard`]'s lock temporary, executes `cb` and then re-acquires it.

> +    ///
> +    /// This can be useful for situations where you may need to do a temporary unlock dance to avoid
> +    /// issues like circular locking dependencies.
> +    ///

Could you add an "# Examples" section here with a simple example?
Thanks!

Otherwise it looks good to me, thanks!

Regards
Boqun

> +    /// 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
>