Branch out the spinlock code into its own header

kernel/defs.h

@@ -2,6 +2,7 @@
 
 #include "riscv.h"
 #include "types.h"
+#include "spinlock.h"
 
 struct buf;
 struct context;
@@ -9,7 +10,6 @@ struct file;
 struct inode;
 struct pipe;
 struct proc;
-struct spinlock;
 struct sleeplock;
 struct stat;
 struct superblock;
@@ -139,14 +139,6 @@ void         procdump(void);
 // swtch.S
 void swtch(struct context *, struct context *);
 
-// spinlock.c
-void acquire(struct spinlock *);
-int  holding(struct spinlock *);
-void initlock(struct spinlock *, char *);
-void release(struct spinlock *);
-void push_off(void);
-void pop_off(void);
-
 // sleeplock.c
 void acquiresleep(struct sleeplock *);
 void releasesleep(struct sleeplock *);

kernel/spinlock.h

@@ -2,10 +2,50 @@
 
 #include "types.h"
 
+struct spinlock;
+
+// spinlock.c
+
+/**
+ * Acquire the lock.
+ * Loops (spins) until the lock is acquired.
+ * Panics if the lock is already held by this cpu.
+ */
+void acquire(struct spinlock *);
+
+/**
+ * Check whether this cpu is holding the lock.
+ * Interrupts must be off.
+ */
+int holding(struct spinlock *);
+
+/**
+ * Initialize spinlock
+ */
+void initlock(struct spinlock *, char *);
+
+/**
+ * Release the lock.
+ * Panics if the lock is not held.
+ */
+void release(struct spinlock *);
+
+/**
+ * @brief push_off/pop_off are like intr_off()/intr_on() except that they are matched:
+ * it takes two pop_off()s to undo two push_off()s.  Also, if interrupts
+ * are initially off, then push_off, pop_off leaves them off.
+ */
+void push_off(void);
+
+/** @copydoc pop_off */
+void pop_off(void);
+
 /** Mutual exclusion spin lock */
 struct spinlock {
   u32 locked; // Is the lock held?
 
+  // NOTE: Perhaps feature gate this?
+
   // For debugging:
   char       *name; // Name of lock.
   struct cpu *cpu;  // The cpu holding the lock.