On Mon, Feb 29, 2016 at 03:47:24PM -0500, Tejun Heo wrote:

If cgroup writeback is in use, inodes can be scheduled for
asynchronous wb switching.  Before 5ff8eaac1636 ("writeback: keep
superblock pinned during cgroup writeback association switches"), this
could race with umount leading to super_block being destroyed while
inodes are pinned for wb switching.  5ff8eaac1636 fixed it by bumping
s_active while wb switches are in flight; however, this allowed
in-flight wb switches to make umounts asynchronous when the userland
expected synchronosity - e.g. fsck immediately following umount may
fail because the device is still busy.

This patch removes the problematic super_block pinning and instead
makes generic_shutdown_super() flush in-flight wb switches.  wb
switches are now executed on a dedicated isw_wq so that they can be
flushed and isw_nr_in_flight keeps track of the number of in-flight wb
switches so that flushing can be avoided in most cases.

Wait a bloody minute.  What's to prevent shrink_dcache_for_umount() from
dirtying more inodes, triggering more of the same?

-	if (!atomic_inc_not_zero(&inode->i_sb->s_active))
-		goto out_unlock;

This would've failed for inodes on superblock in the middle of shutdown;
what's to do the same for the new variant?