protect ip->valid and ip->nlink with sleep lock in iput()

file.h

@@ -14,8 +14,8 @@ struct inode {
   uint dev;           // Device number
   uint inum;          // Inode number
   int ref;            // Reference count
-  struct sleeplock lock;
-  int valid;          // remainder has been read from disk?
+  struct sleeplock lock; // protects everything below here
+  int valid;          // inode has been read from disk?
 
   short type;         // copy of disk inode
   short major;

fs.c

@@ -320,15 +320,17 @@ void
 iput(struct inode *ip)
 {
   acquire(&icache.lock);
-  if(ip->ref == 1 && ip->valid && ip->nlink == 0){
-    // inode has no links and no other references: truncate and free.
+  if(ip->ref == 1){
     acquiresleep(&ip->lock);
+    if(ip->valid && ip->nlink == 0){
+      // inode has no links and no other references: truncate and free.
       release(&icache.lock);
       itrunc(ip);
       ip->type = 0;
       iupdate(ip);
-    acquire(&icache.lock);
       ip->valid = 0;
+      acquire(&icache.lock);
+    }
     releasesleep(&ip->lock);
   }
   ip->ref--;