--- a/src/percpu.rs
+++ b/src/percpu.rs
@@ -21,7 +21,7 @@
     cpu_set::{LogicalCpuId, MAX_CPU_COUNT},
     cpu_stats::{CpuStats, CpuStatsData},
     ptrace::Session,
-    sync::{mcs::McsNode, CleanLockToken},
+    sync::{mcs::McsNode, mcs::McsRawLock, CleanLockToken},
     syscall::debug::SyscallDebugInfo,
 };
 
@@ -66,6 +66,11 @@
     /// `u8::MAX` means unknown (no SRAT or APIC ID not listed).
     pub numa_node: Cell<u8>,
 
+    /// Pointer to the MCS lock this CPU is currently spinning on (for transitive PI).
+    /// `null` when not waiting on any lock. Set in McsRawLock::acquire() before
+    /// entering the spin loop, cleared upon acquisition.
+    pub waiting_on_lock: AtomicPtr<McsRawLock>,
+
     // TODO: Put mailbox queues here, e.g. for TLB shootdown? Just be sure to 128-byte align it
     // first to avoid cache invalidation.
     pub profiling: Option<&'static crate::profiling::RingBuffer>,
@@ -360,6 +365,7 @@
             pi_donated_prio: AtomicU32::new(u32::MAX),
             current_prio: Cell::new(39),
             numa_node: Cell::new(u8::MAX),
+            waiting_on_lock: AtomicPtr::new(core::ptr::null_mut()),
             ptrace_flags: Cell::new(PtraceFlags::empty()),
             ptrace_session: RefCell::new(None),
             inside_syscall: Cell::new(false),