diff --git a/lib/proc.c b/lib/proc.c
index a55c8d6..66bca67 100644
--- a/lib/proc.c
+++ b/lib/proc.c
@@ -2,7 +2,20 @@
 
 struct Cpu cpus[NCPU];
 
+/**
+ * Must be called with interrupts disabled, to prevent race with process being
+ * moved to a different CPU.
+ */
+int cpuid() {
+    int id = read_tp();
+    return id;
+}
+
 /**
  * Return this CPU's cpu struct. Interrupts must be disabled.
  */
-inline struct Cpu *mycpu(void) { return &cpus[read_tp()]; }
+struct Cpu *mycpu(void) {
+    int         id = cpuid();
+    struct Cpu *c = &cpus[id];
+    return c;
+}