diff --git a/FABRIC.md b/FABRIC.md
index baa83f3..d039a95 100644
--- a/FABRIC.md
+++ b/FABRIC.md
@@ -2150,7 +2150,7 @@ on until there is a tick on all three architectures (§16.1, §16.5).*
   > DoE run in a later phase or a dedicated look at the warm-up gate.
   *Refs:* §16.4 (as ruled), §18.4, §21.2, §26.
 
-- [ ] **0.9 — Write the concurrency constraint at the mutex stub.**
+- [x] **0.9 — Write the concurrency constraint at the mutex stub.**
   Add a comment at `src/starkernel/vm/host/shim.c:415` stating that the no-op is correct
   only while nothing in interrupt context mutates shared structure, and that making it a
   real spinlock would deadlock a single hart.
diff --git a/src/starkernel/vm/host/shim.c b/src/starkernel/vm/host/shim.c
index 5cf826c..14e0fb2 100644
--- a/src/starkernel/vm/host/shim.c
+++ b/src/starkernel/vm/host/shim.c
@@ -410,6 +410,14 @@ void sf_mutex_destroy(sf_mutex_t *mutex) {
  * Single-threaded kernel: no contention is possible at the VM level, so
  * acquiring the mutex is always immediate and guaranteed to succeed.
  *
+ * This no-op is correct only as long as nothing running in interrupt
+ * context mutates the structure a given lock protects (FABRIC.md §21.2,
+ * §25.1 item 0.9). If that ever stops being true, making this a real
+ * spinlock is not the fix: a single hart taking an interrupt while
+ * already holding the lock would spin against itself and deadlock. The
+ * correct fix in that case is disabling interrupts around the critical
+ * section, not adding a spinlock here.
+ *
  * @param mutex Ignored.
  */
 void sf_mutex_lock(sf_mutex_t *mutex) {