On Mon 11-06-18 10:20:53, Tejun Heo wrote: > Hello, > > On Mon, Jun 11, 2018 at 06:29:20PM +0200, Jan Kara wrote: > > > 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. Hmm, do you mean like having a per-bdi or even a global mutex that would protect whole wb_shutdown()? Yes, that should work and we could get rid of WB_shutting_down bit as well with that. Just it seems a bit strange to introduce a mutex only to synchronize these two shutdown paths - usually locks protect data structures and in this case we have cgwb_lock for that so it looks like a duplication from a first look. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
