rust_binder: update Process::node_refs to use SpinLock

Unfortunately the current use of a mutex for this lock leads to priority
inversion. Traces have been observed where a process is trying to obtain
this mutex for 22ms, but it's unable to do so because the thread holding
the lock is scheduled out. Since this occurred on a UI thread, that is
an extremely long delay.

Code paths that might sleep under this lock have already been updated in
patches leading up to this one.

Reviewed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260615-binder-noderefs-spin-v3-6-3235f5a3e0a0@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/process.rs

@@ -30,7 +30,7 @@
     sync::{
         aref::ARef,
         lock::{spinlock::SpinLockBackend, Guard},
-        Arc, ArcBorrow, CondVar, CondVarTimeoutResult, Mutex, SpinLock, UniqueArc,
+        Arc, ArcBorrow, CondVar, CondVarTimeoutResult, SpinLock, UniqueArc,
     },
     task::{Pid, Task},
     uaccess::{UserSlice, UserSliceReader},
@@ -455,7 +455,7 @@ pub(crate) struct Process {
     // Node references are in a different lock to avoid recursive acquisition when
     // incrementing/decrementing a node in another process.
     #[pin]
-    node_refs: Mutex<ProcessNodeRefs>,
+    node_refs: SpinLock<ProcessNodeRefs>,
 
     // Work node for deferred work item.
     #[pin]
@@ -510,7 +510,7 @@ fn new(ctx: Arc<Context>, cred: ARef<Credential>) -> Result<Arc<Self>> {
                 cred,
                 inner <- kernel::new_spinlock!(ProcessInner::new(), "Process::inner"),
                 pages <- ShrinkablePageRange::new(&super::BINDER_SHRINKER),
-                node_refs <- kernel::new_mutex!(ProcessNodeRefs::new(), "Process::node_refs"),
+                node_refs <- kernel::new_spinlock!(ProcessNodeRefs::new(), "Process::node_refs"),
                 freeze_wait <- kernel::new_condvar!("Process::freeze_wait"),
                 task: current.group_leader().into(),
                 defer_work <- kernel::new_work!("Process::defer_work"),