Re: [PATCH] bdi: Fix another oops in wb_workfn()

[Jan Kara]

On Mon 11-06-18 09:01:31, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jun 11, 2018 at 11:12:48AM +0200, Jan Kara wrote:
> > However this is wrong and so is the patch. The problem is in
> > cgwb_bdi_unregister() which does cgwb_kill() and thus drops bdi's
> > reference to wb structures before going through the list of wbs again and
> > calling wb_shutdown() on each of them. The writeback structures we are
> > accessing at this point can be already freed in principle like:
> > 
> > CPU1				CPU2
> > 				cgwb_bdi_unregister()
> > 				  cgwb_kill(*slot);
> > 
> > cgwb_release()
> >   queue_work(cgwb_release_wq, &wb->release_work);
> > cgwb_release_workfn()
> > 				  wb = list_first_entry(&bdi->wb_list, ...)
> > 				  spin_unlock_irq(&cgwb_lock);
> >   wb_shutdown(wb);
> >   ...				
> >   kfree_rcu(wb, rcu);
> > 				  wb_shutdown(wb); -> oops use-after-free
> > 
> > I'm not 100% sure how to fix this. wb structures can be at various phases of
> > shutdown (or there may be other external references still existing) when we
> > enter cgwb_bdi_unregister() so I think adding a way for cgwb_bdi_unregister()
> > to wait for standard wb shutdown path to finish is the most robust way.
> > What do you think about attached patch Tejun? So far only compile tested...
> > 
> > Possible problem with it is that now cgwb_bdi_unregister() will wait for
> > all wb references to be dropped so it adds some implicit dependencies to
> > bdi shutdown path. 
> 
> Would something like the following work or am I missing the point
> entirely?

I was pondering the same solution for a while but I think it won't work.
The problem is that e.g. wb_memcg_offline() could have already removed
wb from the radix tree but it is still pending in bdi->wb_list
(wb_shutdown() has not run yet) and so we'd drop reference we didn't get.

								Honza
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 347cc83..359cacd 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -715,14 +715,19 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
>  	WARN_ON(test_bit(WB_registered, &bdi->wb.state));
>  
>  	spin_lock_irq(&cgwb_lock);
> -	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
> -		cgwb_kill(*slot);
> +	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0) {
> +		struct bdi_writeback *wb = *slot;
> +
> +		wb_get(wb);
> +		cgwb_kill(wb);
> +	}
>  
>  	while (!list_empty(&bdi->wb_list)) {
>  		wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
>  				      bdi_node);
>  		spin_unlock_irq(&cgwb_lock);
>  		wb_shutdown(wb);
> +		wb_put(wb);
>  		spin_lock_irq(&cgwb_lock);
>  	}
>  	spin_unlock_irq(&cgwb_lock);
> 
> 
> -- 
> tejun
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR