Hello,

On Mon, Jun 11, 2018 at 06:29:20PM +0200, Jan Kara wrote:

quoted

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.

Yeah, right, so the root cause is that we're walking the wb_list while
holding lock and expecting the object to stay there even after lock is
released.  Hmm... we can use a mutex to synchronize the two
destruction paths.  It's not like they're hot paths anyway.

Thanks.

-- 
tejun