On Thu, Feb 18, 2016 at 08:00:33AM -0500, Tejun Heo wrote:

So, the question is why aren't we just using s_active and draining it
on umount of the last mountpoint.  Because, right now, the behavior is
weird in that we allow umounts to proceed but then let the superblock
hang onto the block device till s_active is drained.  This really
should be synchronous.

This really should not.  First of all, umount -l (or exit of the last
namespace user, for that matter) can leave you with actual fs shutdown
postponed until some opened files get closed.  Nothing synchronous about
that.

If you need details on s_active/s_umount/etc., I can give you a braindump,
but I suspect your real question is a lot more specific.  Details, please...