On Thu, 3 Sep 2009, Eric Dumazet wrote: > Point is we cannot deal with RCU quietness before disposing the slab cache, > (if SLAB_DESTROY_BY_RCU was set on the cache) since this disposing *will* > make call_rcu() calls when a full slab is freed/purged. There is no need to do call_rcu calls for frees at that point since objects are no longer in use. We could simply disable SLAB_DESTROY_BY_RCU for the final clearing of caches. > And when RCU grace period is elapsed, the callback *will* need access to > the cache we want to dismantle. Better to not have kfreed()/poisoned it... But going through the RCU period is pointless since no user of the cache remains. > I believe you mix two RCU uses here. > > 1) The one we all know, is use normal caches (!SLAB_DESTROY_BY_RCU) > (or kmalloc()), and use call_rcu(... kfree_something) > > In this case, you are 100% right that the subsystem itself has > to call rcu_barrier() (or respect whatever self-synchro) itself, > before calling kmem_cache_destroy() > > 2) The SLAB_DESTROY_BY_RCU one. > > Part of cache dismantle needs to call rcu_barrier() itself. > Caller doesnt have to use rcu_barrier(). It would be a waste of time, > as kmem_cache_destroy() will refill rcu wait queues with its own stuff. The dismantling does not need RCU since there are no operations on the objects in progress. So simply switch DESTROY_BY_RCU off for close. --- mm/slub.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) Index: linux-2.6/mm/slub.c =================================================================== --- linux-2.6.orig/mm/slub.c 2009-09-03 10:14:51.000000000 -0500 +++ linux-2.6/mm/slub.c 2009-09-03 10:18:32.000000000 -0500 @@ -2594,9 +2594,9 @@ static inline int kmem_cache_close(struc */ void kmem_cache_destroy(struct kmem_cache *s) { - if (s->flags & SLAB_DESTROY_BY_RCU) - rcu_barrier(); down_write(&slub_lock); + /* Stop deferring frees so that we can immediately free structures */ + s->flags &= ~SLAB_DESTROY_BY_RCU; s->refcount--; if (!s->refcount) { list_del(&s->list); -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html