Re: [PATCH 09/12] KVM: MMU: introduce pte-list lockless walker

[Gleb Natapov <gleb@redhat.com>]

On Thu, Aug 29, 2013 at 05:31:42PM +0800, Xiao Guangrong wrote:
> > As Documentation/RCU/whatisRCU.txt says:
> > 
> >         As with rcu_assign_pointer(), an important function of
> >         rcu_dereference() is to document which pointers are protected by
> >         RCU, in particular, flagging a pointer that is subject to changing
> >         at any time, including immediately after the rcu_dereference().
> >         And, again like rcu_assign_pointer(), rcu_dereference() is
> >         typically used indirectly, via the _rcu list-manipulation
> >         primitives, such as list_for_each_entry_rcu().
> > 
> > The documentation aspect of rcu_assign_pointer()/rcu_dereference() is
> > important. The code is complicated, so self documentation will not hurt.
> > I want to see what is actually protected by rcu here. Freeing shadow
> > pages with call_rcu() further complicates matters: does it mean that
> > shadow pages are also protected by rcu? 
> 
> Yes, it stops shadow page to be freed when we do write-protection on
> it.
> 
Yeah, I got the trick, what I am saying that we have a data structure
here protected by RCU, but we do not use RCU functions to access it...
BTW why not allocate sp->spt from SLAB_DESTROY_BY_RCU cache too? We may
switch write protection on a random spt occasionally if page is deleted
and reused for another spt though. For last level spt it should not be a
problem and for non last level we have is_last_spte() check in
__rmap_write_protect_lockless(). Can it work?

--
			Gleb.