From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755715Ab0CDQeY (ORCPT ); Thu, 4 Mar 2010 11:34:24 -0500 Received: from cantor2.suse.de ([195.135.220.15]:45496 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755432Ab0CDQeW (ORCPT ); Thu, 4 Mar 2010 11:34:22 -0500 Date: Fri, 5 Mar 2010 03:34:18 +1100 From: Nick Piggin To: Peter Zijlstra Cc: Miao Xie , David Rientjes , Lee Schermerhorn , Paul Menage , Linux-Kernel , Linux-MM , tglx Subject: Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems_allowed Message-ID: <20100304163417.GS8653@laptop> References: <4B8E3F77.6070201@cn.fujitsu.com> <20100304033017.GN8653@laptop> <1267714704.25158.199.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1267714704.25158.199.camel@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 04, 2010 at 03:58:24PM +0100, Peter Zijlstra wrote: > On Thu, 2010-03-04 at 14:30 +1100, Nick Piggin wrote: > > > > Thanks for working on this. However, rwlocks are pretty nasty to use > > when you have short critical sections and hot read-side (they're twice > > as heavy as even spinlocks in that case). > > Should we add a checkpatch.pl warning for them? Yes I think it could be useful. Most people agree rwlock is *almost* always the wrong thing to do. Or at least, they can easily be used wrongly because they seem like a great idea for read-mostly data. > > There really rarely is a good case for using rwlock_t, for as you say > they're a pain and often more expensive than a spinlock_t, and if > possible RCU has the best performance. Yep. Not to mention they starve writers (and don't FIFO like spinlocks). Between normal spinlocks, RCU, percpu, and seqlocks, there's not much room for rwlocks. Even tasklist lock should be RCUable if the effort is put into it.