On Mon, Nov 25, 2013 at 06:59:24PM +0800, Xiao Guangrong wrote: > > 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). Oh, let me have a look, I don't think I've really looked at that yet. > > 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. Then its broken, SLAB_DESTROY_BY_RCU rarely does what people expect it to; which is why I wrote that honkin' huge comment right next to it. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
