[PATCH] rust: devres: ensure revocation is complete before device finishes unbinding

[Danilo Krummrich]

Now that the revocation Completion is in place, also address the
symmetric case. When Devres::drop() wins the is_available swap and the
devres callback loses, the callback returns to devres_release_all()
without waiting. This means device unbinding can complete while
Devres::drop() is still executing drop_in_place() on another CPU, which
is a problem if T's destructor accesses device state.

Make the synchronization bidirectional. Whichever side performs
drop_in_place() signals the Completion, and the other side waits.

This does not reintroduce the nested Devres deadlock fixed by commit
ba268514ea14 ("rust: devres: fix race condition due to nesting"),
because that deadlock was caused by drop waiting for the release
callback to return (the old 'devm' Completion). Here, both sides only
wait for drop_in_place() to finish, which completes within the current
call chain. The Arc<Inner<T>> keeps the Inner allocation alive
independently.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: ba268514ea14 ("rust: devres: fix race condition due to nesting")
Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
---
 rust/kernel/devres.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 11d862f1e6de..f112c7e8bc3b 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -264,6 +264,11 @@ fn data(&self) -> &Revocable<T> {
 
         if inner.data.revoke() {
             inner.revocation.complete_all();
+        } else {
+            // Devres::drop() is concurrently revoking; wait for it to finish `drop_in_place()`
+            // before returning to `devres_release_all()`, ensuring `T` is fully torn down before
+            // the device finishes unbinding.
+            inner.revocation.wait_for_completion();
         }
     }
 
@@ -364,6 +369,8 @@ fn drop(&mut self) {
         // SAFETY: When `drop` runs, it is guaranteed that nobody is accessing the revocable data
         // anymore, hence it is safe not to wait for the grace period to finish.
         if unsafe { self.data().revoke_nosync() } {
+            self.inner.revocation.complete_all();
+
             // We revoked `self.data` before devres did, hence try to remove it.
             if self.remove_node() {
                 // SAFETY: In `Self::new` we have taken an additional reference count of `self.data`

base-commit: 6cb8c4e26a3684c9df382a350f06bfbe2a197e5e
-- 
2.54.0