Re: [PATCH 08/10] block: Fix oops in locked_inode_to_wb_and_lock_list()

[Jan Kara <jack@suse.cz>]

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@suse.com>
SUSE Labs, CR