On Sun 12-02-17 13:40:27, Tejun Heo wrote: > Hello, Jan. > > On Thu, Feb 09, 2017 at 01:44:31PM +0100, Jan Kara wrote: > > When block device is closed, we call inode_detach_wb() in __blkdev_put() > > which sets inode->i_wb to NULL. That is contrary to expectations that > > inode->i_wb stays valid once set during the whole inode's lifetime and > > leads to oops in wb_get() in locked_inode_to_wb_and_lock_list() because > > inode_to_wb() returned NULL. > > > > The reason why we called inode_detach_wb() is not valid anymore though. > > BDI is guaranteed to stay along until we call bdi_put() from > > bdev_evict_inode() so we can postpone calling inode_detach_wb() to that > > moment. A complication is that i_wb can point to non-root wb_writeback > > structure and in that case we do need to clean it up as bdi_unregister() > > blocks waiting for all non-root wb_writeback references to get dropped. > > Thus this i_wb reference could block device removal e.g. from > > __scsi_remove_device() (which indirectly ends up calling > > bdi_unregister()). We cannot rely on block device inode to go away soon > > (and thus i_wb reference to get dropped) as the device may got > > hot-removed e.g. under a mounted filesystem. We deal with these issues > > by switching block device inode from non-root wb_writeback structure to > > bdi->wb when needed. Since this is rather expensive (requires > > synchronize_rcu()) we do the switching only in del_gendisk() when we > > know the device is going away. > > So, the only reason cgwb_bdi_destroy() is synchronous is because bdi > destruction was synchronous. Now that bdi is properly reference > counted and can be decoupled from gendisk / q destruction, I can't > think of a reason to keep cgwb destruction synchronous. Switching > wb's on destruction is kinda clumsy and it almost always hurts to > expose synchronize_rcu() in userland visible paths. > > Wouldn't something like the following work? > > * Remove bdi->usage_cnt and the synchronous waiting in > cgwb_bdi_destroy(). > > * Instead, make cgwb's hold bdi->refcnt and put it from > cgwb_release_workfn(). > > Then, we don't have to switch during shutdown and can just let things > drain. At first sight this looks workable and would mean less special code so I like it. I'll experiment with it and see how it works out. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
