Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751415AbZL1C6x (ORCPT ); Sun, 27 Dec 2009 21:58:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751282AbZL1C6w (ORCPT ); Sun, 27 Dec 2009 21:58:52 -0500 Received: from e23smtp04.au.ibm.com ([202.81.31.146]:52329 "EHLO e23smtp04.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751210AbZL1C6v (ORCPT ); Sun, 27 Dec 2009 21:58:51 -0500 Date: Mon, 28 Dec 2009 08:28:39 +0530 From: Balbir Singh To: KAMEZAWA Hiroyuki Cc: Peter Zijlstra , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "minchan.kim@gmail.com" , cl@linux-foundation.org Subject: Re: [RFC PATCH] asynchronous page fault. Message-ID: <20091228025839.GF3601@balbir.in.ibm.com> Reply-To: balbir@linux.vnet.ibm.com References: <20091225105140.263180e8.kamezawa.hiroyu@jp.fujitsu.com> <1261912796.15854.25.camel@laptop> <20091228005746.GE3601@balbir.in.ibm.com> <20091228100514.ec6f9949.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20091228100514.ec6f9949.kamezawa.hiroyu@jp.fujitsu.com> User-Agent: Mutt/1.5.20 (2009-08-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1839 Lines: 46 * KAMEZAWA Hiroyuki [2009-12-28 10:05:14]: > On Mon, 28 Dec 2009 06:27:46 +0530 > Balbir Singh wrote: > > > * Peter Zijlstra [2009-12-27 12:19:56]: > > > > > Your changelog states as much. > > > > > > "Even if RB-tree rotation occurs while we walk tree for look-up, we just > > > miss vma without oops." > > > > > > However, since this is the case, do we still need the > > > rcu_assign_pointer() conversion your patch does? All I can see it do is > > > slow down all RB-tree users, without any gain. > > > > Don't we need the rcu_assign_pointer() on the read side primarily to > > make sure the pointer is still valid and assignments (writes) are not > > re-ordered? Are you suggesting that the pointer assignment paths be > > completely atomic? > > > >From following reasons. > - What we have to avoid is not to touch unkonwn memory via broken pointer. > This is speculative look up and can miss vmas. So, even if tree is broken, > there is no problem. Broken pointer which points to places other than rb-tree > is problem. Exactly! > - rb-tree's rb_left and rb_right don't points to memory other than > rb-tree. (or NULL) And vmas are not freed/reused while rcu_read_lock(). > Then, we don't dive into unknown memory. > - Then, we can skip rcu_assign_pointer(). > We can, but the data being on read-side is going to be out-of-date more than without the use of rcu_assign_pointer(). Do we need variants like to rcu_rb_next() to avoid overheads for everyone? -- Balbir -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/