On Wed, Aug 12, 2009 at 08:26:39AM -0700, Paul E. McKenney wrote: > On Wed, Aug 12, 2009 at 05:15:59PM +0300, Michael S. Tsirkin wrote: > > On Wed, Aug 12, 2009 at 07:11:07AM -0700, Paul E. McKenney wrote: > > > 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. > > > > Right. I don't use any of these though. > > > > > If you -don't- use rcu_read_lock(), then you are pretty much restricted > > > to adding data, but never removing it. > > > > > > Make sense? ;-) > > > > Since I only access data from a workqueue, I replaced synchronize_rcu > > with workqueue flush. That's why I don't need rcu_read_lock. > > Well, you -do- need -something- that takes on the role of rcu_read_lock(), > and in your case you in fact actually do. Your equivalent of > rcu_read_lock() is the beginning of execution of a workqueue item, and > the equivalent of rcu_read_unlock() is the end of execution of that same > workqueue item. Implicit, but no less real.
Well put. I'll add this to comments in my code. > If a couple more uses like this show up, I might need to add this to > Documentation/RCU. ;-) > > Thanx, Paul _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
