BUG in writeback_inodes()?

BUG in writeback_inodes()?

[Kirill Korotaev]

Hello All,

It looks like there is a small race bug in writeback_inodes()
Have a look at this 2 call chains:

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().

Am I correct with it?

Kirill

Re: BUG in writeback_inodes()?

[Chris Mason]

On Mon, 2004-09-13 at 12:01, Kirill Korotaev wrote:

> 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().
> 
> Am I correct with it?

Hmmm, sure looks that way.  Seems like it should be enough to switch to
list_del_init in deactivate_super, and then check for
list_empty(sb->s_list) in writeback_inodes.

-chris