Hello, On Tue, Feb 21, 2017 at 06:09:54PM +0100, Jan Kara wrote: > @@ -726,14 +718,6 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi) > } > > spin_unlock_irq(&cgwb_lock); > - > - /* > - * All cgwb's and their congested states must be shutdown and > - * released before returning. Drain the usage counter to wait for > - * all cgwb's and cgwb_congested's ever created on @bdi. > - */ > - atomic_dec(&bdi->usage_cnt); > - wait_event(cgwb_release_wait, !atomic_read(&bdi->usage_cnt)); > } Hmm... I'm not sure about wb_shutdown() synchronization. If you look at the function, it's allowed to be called multiple times but doesn't synchronize the end of the operation. With usage_cnt, it was okay because cgwb_bdi_destroy() would have waited until everything is finished via usage_cnt, but with that gone, we can have a race like the following. A B a cgroup gets removed a cgwb starts to get destroyed it starts wb_shutdown() bdi starts getting destroyed calls cgwb_bdi_destroy() calls wb_shutdown() on the same cgwb but it returns because it lost to wb_shutdown() is still in progress A's wb_shutdown() bdi destruction proceeds Oops. So, I think we need to make sure that wb_shutdown()'s are properly synchronized from start to end to get rid of the usage_cnt waiting. Thanks. -- tejun
