Redesign percpu<> const-ness and general percpu cleanup

percpu acts like a T* const, so a const percpu should (also) be like a T* const, *not* like a const T*, which is what the code previously did (remember, C types are inside-out! except when they aren't). This, for example, makes it sane to pass a const percpu reference. This also makes percpu non-copyable and non-assignable and makes stuff private.

Redesign percpu<> const-ness and general percpu cleanup
2743888d · Austin Clements · 23af7cf9 · 2743888d
--- a/include/percpu.hh
+++ b/include/percpu.hh
@@ -4,31 +4,38 @@ extern int mycpuid(void);
 template <typename T>
 struct percpu {
-  const T* operator->() const {
+  percpu() = default;
+  percpu(const percpu &o) = delete;
+  percpu(percpu &&o) = delete;
+  percpu &operator=(const percpu &o) = delete;
+  T* get() const {
    return cpu(mycpuid());
  }
-  T* operator->() {
+  T* operator->() const {
    return cpu(mycpuid());
  }
-  T& operator*() {
+  T& operator*() const {
    return *cpu(mycpuid());
  }
-  T& operator[](int id) { 
+  T& operator[](int id) const {
-    return *cpu(id);
-  }
-  const T& operator[](int id) const { 
    return *cpu(id);
  }
-  T* cpu(int id) {
+private:
+  T* cpu(int id) const {
    return &pad_[id].v_;
  }
-  struct {
+  // percpu acts like a T* const, but since it's actually storing the
+  // data directly, we have to strip the const-ness away from the data
+  // being stored.  This lets const members return non-const pointers
+  // to this data, just like a T* const.
+  mutable struct {
    T v_ __mpalign__;
    __padout__;
  } pad_[NCPU];