[PATCH] Improve sget() performance

sget() is pessimistic and assumes it won't find a superblock - and as a
result it always pre-allocates a new one before even searching for an
existing entry.  If it finds an existing one, it free's the extra one it
just allocated.

Especially with /proc usage it usually _does_ find an existing
superblock, so this is all extra work.  It should allocate a new one
only if it couldn't find an old one (and re-search).

The same way as iget() does it.

fs/super.c

@@ -231,13 +231,10 @@ struct super_block *sget(struct file_system_type *type,
 			int (*set)(struct super_block *,void *),
 			void *data)
 {
-	struct super_block *s = alloc_super();
+	struct super_block *s = NULL;
 	struct list_head *p;
 	int err;
 
-	if (!s)
-		return ERR_PTR(-ENOMEM);
-
 retry:
 	spin_lock(&sb_lock);
 	if (test) list_for_each(p, &type->fs_supers) {
@@ -247,9 +244,18 @@ struct super_block *sget(struct file_system_type *type,
 			continue;
 		if (!grab_super(old))
 			goto retry;
+		if (s)
 			destroy_super(s);
 		return old;
 	}
+	if (!s) {
+		spin_unlock(&sb_lock);
+		s = alloc_super();
+		if (!s)
+			return ERR_PTR(-ENOMEM);
+		goto retry;
+	}
+		
 	err = set(s, data);
 	if (err) {
 		spin_unlock(&sb_lock);