From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU Date: Tue, 7 Oct 2008 07:38:18 -0700 Message-ID: <20081007143818.GC6384@linux.vnet.ibm.com> References: <20081006185026.GA10383@minyard.local> <48EA8197.6080502@cosmosbay.com> <1223367480.26330.7.camel@lappy.programming.kicks-ass.net> <48EB2AE3.3080200@cosmosbay.com> <48EB6EE4.8030703@linux-foundation.org> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Dumazet , Peter Zijlstra , minyard@acm.org, Linux Kernel , netdev@vger.kernel.org, shemminger@vyatta.com To: Christoph Lameter Return-path: Received: from e4.ny.us.ibm.com ([32.97.182.144]:59885 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752640AbYJGOpJ (ORCPT ); Tue, 7 Oct 2008 10:45:09 -0400 Content-Disposition: inline In-Reply-To: <48EB6EE4.8030703@linux-foundation.org> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Oct 07, 2008 at 09:15:00AM -0500, Christoph Lameter wrote: > Eric Dumazet wrote: > >>> Or just add SLAB_DESTROY_BY_RCU to slab creation in proto_register() > >>> for "struct proto udp_prot/udpv6_prot" so that kmem_cache_free() done > >>> in sk_prot_free() can defer freeing to RCU... > >> > >> Be careful!, SLAB_DESTROY_BY_RCU just means the slab page gets > >> RCU-freed, this means that slab object pointers stay pointing to valid > >> memory, but it does _NOT_ mean those slab objects themselves remain > >> valid. > >> > >> The slab allocator is free to re-use those objects at any time - > >> irrespective of the rcu-grace period. Therefore you will have to be able > >> to validate that the object you point to is indeed the object you > >> expect, otherwise strange and wonderful things will happen. > >> > > Thanks for this clarification. I guess we really need a rcu head then :) > > No you just need to make sure that the object you located is still active > (f.e. refcount > 0) and that it is really a match (hash pointers may be > updated asynchronously and therefore point to the object that has been reused > for something else). In some cases, you might be able to not care, but yes, most of the time, you will need to validate the object. > Generally it is advisable to use SLAB_DESTROY_BY_RCU because it preserves the > cache hot advantages of the objects. Regular RCU freeing will let the object > expire for a tick or so which will result in the cacheline cooling down. And SLAB_DESTROY_BY_RCU guarantees that the type of the object will remain the same during any given RCU read-side critical section. Thanx, Paul