[RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Al Viro]

On Fri, Jun 08, 2012 at 03:25:50PM -0700, Andrew Morton wrote:

> A neater implementation might be to add a kmem_cache* argument to
> unregister_filesystem().  If that is non-NULL, unregister_filesystem()
> does the rcu_barrier() and destroys the cache.  That way we get to
> delete (rather than add) a bunch of code from all filesystems and new
> and out-of-tree filesystems cannot forget to perform the rcu_barrier().

There's often enough more than one cache, so that one is no-go.