From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754917AbaIQJgs (ORCPT ); Wed, 17 Sep 2014 05:36:48 -0400 Received: from cantor2.suse.de ([195.135.220.15]:54784 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753561AbaIQJgr (ORCPT ); Wed, 17 Sep 2014 05:36:47 -0400 Message-ID: <1410946476.9389.32.camel@linux-t7sj.site> Subject: Re: [PATCH v3] locking/rwsem: Avoid double checking before try acquiring write lock From: Davidlohr Bueso To: Jason Low Cc: Peter Zijlstra , Ingo Molnar , Tim Chen , Peter Hurley , linux-kernel@vger.kernel.org, Aswin Chandramouleeswaran , Chegu Vinod Date: Wed, 17 Sep 2014 11:34:37 +0200 In-Reply-To: <1410913017.2447.22.camel@j-VirtualBox> References: <1410913017.2447.22.camel@j-VirtualBox> Organization: SUSE Labs Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2014-09-16 at 17:16 -0700, Jason Low wrote: > Commit 9b0fc9c09f1b checks for if there are known active lockers > in order to avoid write trylocking using expensive cmpxchg() when > it likely wouldn't get the lock. Ah, I remember you had this one in your queue for some time :) > However, a subsequent patch was added such that we directly > check for sem->count == RWSEM_WAITING_BIAS right before trying > that cmpxchg(). Thus, commit 9b0fc9c09f1b now just adds overhead. > This patch modifies it so that we only do a check for if > count == RWSEM_WAITING_BIAS. > > Also, add a comment on why we do an "extra check" of count > before the cmpxchg(). heh, so people don't try to remove the "redundant" check! > Cc: Peter Hurley > Cc: Tim Chen Acked-by: Davidlohr Bueso > Signed-off-by: Jason Low