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

[Tejun Heo <tj@kernel.org>]

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?

Thanks.

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