On Wed, Aug 12, 2009 at 04:25:40PM +0300, Michael S. Tsirkin wrote: > On Wed, Aug 12, 2009 at 09:01:35AM -0400, Gregory Haskins wrote: > > I think I understand what your comment above meant: You don't need to > > do synchronize_rcu() because you can flush the workqueue instead to > > ensure that all readers have completed. > > Yes. > > > But if thats true, to me, the > > rcu_dereference itself is gratuitous, > > Here's a thesis on what rcu_dereference does (besides documentation): > > reader does this > > A: sock = n->sock > B: use *sock > > Say writer does this: > > C: newsock = allocate socket > D: initialize(newsock) > E: n->sock = newsock > F: flush > > > On Alpha, reads could be reordered. So, on smp, command A could get > data from point F, and command B - from point D (uninitialized, from > cache). IOW, you get fresh pointer but stale data. > So we need to stick a barrier in there. > > > and that pointer is *not* actually > > RCU protected (nor does it need to be). > > Heh, if readers are lockless and writer does init/update/sync, > this to me spells rcu. If you are using call_rcu(), synchronize_rcu(), or one of the similar primitives, then you absolutely need rcu_read_lock() and rcu_read_unlock(), or one of the similar pairs of primitives. If you -don't- use rcu_read_lock(), then you are pretty much restricted to adding data, but never removing it. Make sense? ;-) Thanx, Paul -- 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
