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

[Linus Torvalds]

On Fri, Jun 8, 2012 at 2:28 PM, Kirill A. Shutemov
<kirill.shutemov at linux.intel.com> wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
>
> There's no reason to call rcu_barrier() on every deactivate_locked_super().
> We only need to make sure that all delayed rcu free inodes are flushed
> before we destroy related cache.
>
> Removing rcu_barrier() from deactivate_locked_super() affects some
> fas paths. E.g. on my machine exit_group() of a last process in IPC
> namespace takes 0.07538s. rcu_barrier() takes 0.05188s of that time.

I think we should just delete it.

kmem_cache_destroy() (at least for SLUB) already has:

                if (s->flags & SLAB_DESTROY_BY_RCU)
                        rcu_barrier();

in it. But I think it's too late - it gets called *after* we do
kmem_cache_close(), and I get the feeling that we should do it before.

Shouldn't that be sufficient? And if other slab allocators don't have
this, we should add it to them too.

Hmm?

              Linus