[PATCH 08/12] superblock: introduce per-sb cache shrinker infrastructure

[Al Viro]

On Sat, Jun 04, 2011 at 10:08:48AM -0400, Christoph Hellwig wrote:
> On Sat, Jun 04, 2011 at 11:52:12AM +1000, Dave Chinner wrote:
> > I wanted to put it early on in the unmount path so that the shrinker
> > was guaranteed to be gone before evict_inodes() was called. That
> > would mean that it is obviously safe to remove the iprune_sem
> > serialisation in that function.
> 
> The iprune_sem removal is fine as soon as you have a per-sb shrinker
> for the inodes which keeps an active reference on the superblock until
> all the inodes are evicted.

I really don't like that.  Stuff keeping active refs, worse yet doing that
asynchronously...  Shrinkers should *not* do that.  Just grab a passive
ref (i.e. bump s_count), try grab s_umount (shared) and if that thing still
has ->s_root while we hold s_umount, go ahead.  Unregister either at the
end of generic_shutdown_super() or from deactivate_locked_super(), between
the calls of ->kill_sb() and put_filesystem().