[PATCH] Fix of race in writeback_inodes()

This patch fixes race in writeback_inodes() described below:

writeback_inodes()
{
     ....
     sb->s_count++;
     spin_unlock(&sb_lock);
     ....
     spin_lock(&sb_lock);
     if (__put_super(sb))        <<< X
         goto restart;
     }
}

deactivate_super()
{
     fs->kill_sb(s);
         kill_block_super(sb)
             generic_shutdown_super(sb)
                     spin_lock(&sb_lock);
                     list_del(&sb->s_list);    <<< Y
                     spin_unlock(&sb_lock);
     ....
     put_super(s);
             spin_lock(&sb_lock);
             __put_super(sb);            <<< Z
             spin_unlock(&sb_lock);
}

The problem with it is that writeback_inodes() supposes that if
__put_super() returns 0 then no super block was deleted from the list and
we can safely traverse sb list further.

But as it is obvious from the deactivate_super() it's not actually true.
because at point Y we delete super block from the list and drop the lock.
We do __put_super() very much later...  So we can find sb with poisoned
sb->s_list at point X and we won't be the last sb reference holders.  The
last reference will be dropped in point Z.

So in case of the following sequence of execution Y -> X -> Z we'll get an
oops after point X in writeback_inodes().

This patch introduces __put_super_and_need_restart() function which allows
safe traversing of sb list.  I'll send a couple of patches later which
remove O(n^2) algos and using this function.
Signed-Off-By: Kirill Korotaev <dev@sw.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/fs-writeback.c

@@ -423,7 +423,7 @@ writeback_inodes(struct writeback_control *wbc)
 				up_read(&sb->s_umount);
 			}
 			spin_lock(&sb_lock);
-			if (__put_super(sb))
+			if (__put_super_and_need_restart(sb))
 				goto restart;
 		}
 		if (wbc->nr_to_write <= 0)

fs/super.c

@@ -116,6 +116,27 @@ int __put_super(struct super_block *sb)
 	return ret;
 }
 
+/*
+ * Drop a superblock's refcount.
+ * Returns non-zero if the superblock is about to be destroyed and
+ * at least is already removed from super_blocks list, so if we are
+ * making a loop through super blocks then we need to restart.
+ * The caller must hold sb_lock.
+ */
+int __put_super_and_need_restart(struct super_block *sb)
+{
+	/* check for race with generic_shutdown_super() */
+	if (list_empty(&sb->s_list)) {
+		/* super block is removed, need to restart... */
+		__put_super(sb);
+		return 1;
+	}
+	/* can't be the last, since s_list is still in use */
+	sb->s_count--;
+	BUG_ON(sb->s_count == 0);
+	return 0;
+}
+
 /**
  *	put_super	-	drop a temporary reference to superblock
  *	@s: superblock in question
@@ -229,7 +250,8 @@ void generic_shutdown_super(struct super_block *sb)
 		unlock_super(sb);
 	}
 	spin_lock(&sb_lock);
-	list_del(&sb->s_list);
+	/* should be initialized for __put_super_and_need_restart() */
+	list_del_init(&sb->s_list);
 	list_del(&sb->s_instances);
 	spin_unlock(&sb_lock);
 	up_write(&sb->s_umount);
@@ -282,7 +304,7 @@ struct super_block *sget(struct file_system_type *type,
 	}
 	s->s_type = type;
 	strlcpy(s->s_id, type->name, sizeof(s->s_id));
-	list_add(&s->s_list, super_blocks.prev);
+	list_add_tail(&s->s_list, &super_blocks);
 	list_add(&s->s_instances, &type->fs_supers);
 	spin_unlock(&sb_lock);
 	get_filesystem(type);

include/linux/fs.h

@@ -1149,6 +1149,7 @@ struct super_block *sget(struct file_system_type *type,
 struct super_block *get_sb_pseudo(struct file_system_type *, char *,
 			struct super_operations *ops, unsigned long);
 int __put_super(struct super_block *sb);
+int __put_super_and_need_restart(struct super_block *sb);
 void unnamed_dev_init(void);
 
 /* Alas, no aliases. Too much hassle with bringing module.h everywhere */