Support static construction of lockstat-enabled locks

This works by lazily allocating a lock's klockstat structure when the lock is first acquired.

Support static construction of lockstat-enabled locks
da450a4f · Austin Clements · b6308520 · da450a4f · da450a4f
--- a/include/spinlock.h
+++ b/include/spinlock.h
 #pragma once
 #include "lockstat.h"

+#if LOCKSTAT
+extern klockstat klockstat_lazy;
+#endif
+
 // Mutual exclusion lock.
 struct spinlock {
  u32 locked;       // Is the lock held?
@@ -29,16 +33,15 @@ struct spinlock {
 #endif
  { }

-  // Create a spinlock without lockstat tracking.  This is constexpr,
-  // so it can be used for global spinlocks without incurring a static
-  // constructor.
-  constexpr spinlock(const char *name)
+  // Create a spinlock.  This is constexpr, so it can be used for
+  // global spinlocks without incurring a static constructor.
+  constexpr spinlock(const char *name, bool lockstat = false)
    : locked(0)
 #if SPINLOCK_DEBUG
    , name(name), cpu(nullptr), pcs{}
 #endif
 #if LOCKSTAT
-    , stat(nullptr)
+    , stat(lockstat ? nullptr : &klockstat_lazy)
 #endif
  { }
 };

--- a/kernel/spinlock.cc
+++ b/kernel/spinlock.cc
@@ -13,8 +13,14 @@
 #include "major.h"

 #if LOCKSTAT
+// The klockstat structure pointed to by spinlocks that want lockstat,
+// but have never been acquired.
+struct klockstat klockstat_lazy("<lazy>");
+
 static int lockstat_enable;

+void lockstat_init(struct spinlock *lk, bool lazy);
+
 static inline struct cpulockstat *
 mylockstat(struct spinlock *lk)
 {
@@ -49,8 +55,11 @@ locking(struct spinlock *lk)
 #endif

 #if LOCKSTAT
-  if (lockstat_enable && lk->stat != nullptr)
+  if (lockstat_enable && lk->stat != nullptr) {
+    if (lk->stat == &klockstat_lazy)
+      lockstat_init(lk, true);
    mylockstat(lk)->locking_ts = rdtsc();
+  }
 #endif

  mtlock(lk);
@@ -128,12 +137,21 @@ klockstat::klockstat(const char *name) :
 };

 void
-lockstat_init(struct spinlock *lk)
+lockstat_init(struct spinlock *lk, bool lazy)
 {
-  lk->stat = new klockstat(lk->name);
-  if (lk->stat == 0)
+  klockstat *ls = new klockstat(lk->name);
+  if (!ls)
    return;

+  if (lazy) {
+    if (!__sync_bool_compare_and_swap(&lk->stat, &klockstat_lazy, ls)) {
+      delete ls;
+      return;
+    }
+  } else {
+    lk->stat = ls;
+  }
+
  acquire(&lockstat_lock);
  LIST_INSERT_HEAD(&lockstat_list, lk->stat, link);
  release(&lockstat_lock);
@@ -256,7 +274,7 @@ initlock(struct spinlock *lk, const char *name, int lockstat)
 #if LOCKSTAT
  lk->stat = (struct klockstat*) nullptr;
  if (lockstat)
-    lockstat_init(lk);
+    lockstat_init(lk, false);
 #endif
  lk->locked = 0;
 }