Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

Xiao Guangrong
2013-11-25T10:59:36Z

Hi Peter,

On 11/25/2013 05:31 PM, Peter Zijlstra wrote:
> On Fri, Nov 22, 2013 at 05:14:29PM -0200, Marcelo Tosatti wrote:
>> Also, there is no guarantee of termination (as long as sptes are
>> deleted with the correct timing). BTW, can't see any guarantee of
>> termination for rculist nulls either (a writer can race with a lockless
>> reader indefinately, restarting the lockless walk every time).
>
> What's an rculist null?

I guess Marcelo was talking about rculist_nulls.h
(Documentation/RCU/rculist_nulls.txt).

> rculists have regular termination conditions,
> they'll reach the end (also head, due to circular etc..) in N steps,
> where N is the number of elements.
>
> Of course you can keep adding elements to protract this situation, but
> you'll run out of memory eventually -- you also have to make sure to
> insert them after the element being read, otherwise the iteration will
> simply miss them.
>
> Deleting an element preserves the element itself -- it has to be
> RCU-freed to be part of an rculist, and the element also preserves its
> fwd link, so any iterator will either not see the element, or if they're
> at the element, they'll continue their iteration as normal (rculist
> doesn't have backward iterators).
>
> A deleted element may not be re-used before an RCU grace period has
> lapsed. Same as for freeing such an element. So if you want to move an
> rculist element you need to:
>
> list_del_rcu()
> synchronize_rcu();
> list_add_rcu();
>
> Or use the list_splice_init_rcu() primitive which also explicitly takes
> a @sync argument.

Thanks for your detailed explanation, Peter!

What about if the element is allocated from SLAB_DESTROY_BY_RCU slab? That
means the element may be reused while do iteration.