From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752135AbcFODri (ORCPT ); Tue, 14 Jun 2016 23:47:38 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:15362 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751113AbcFODrg (ORCPT ); Tue, 14 Jun 2016 23:47:36 -0400 X-IBM-Helo: d23dlp02.au.ibm.com X-IBM-MailFrom: xinhui.pan@linux.vnet.ibm.com X-IBM-RcptTo: linux-arch@vger.kernel.org;linux-kernel@vger.kernel.org Date: Wed, 15 Jun 2016 11:47:21 +0800 From: xinhui User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.8.0 MIME-Version: 1.0 To: Will Deacon CC: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, arnd@arndb.de, waiman.long@hp.com, peterz@infradead.org Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian References: <1464862148-5672-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <20160608092244.GA27029@arm.com> <575FA024.7060608@linux.vnet.ibm.com> <20160614104032.GE19407@arm.com> In-Reply-To: <20160614104032.GE19407@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16061503-0052-0000-0000-000001A2C92E X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16061503-0053-0000-0000-00000632119F Message-Id: <5760CFC9.6020006@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-06-15_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1604210000 definitions=main-1606150042 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016年06月14日 18:40, Will Deacon wrote: > On Tue, Jun 14, 2016 at 02:11:48PM +0800, xinhui wrote: >> >> On 2016年06月08日 17:22, Will Deacon wrote: >>> On Thu, Jun 02, 2016 at 06:09:08PM +0800, Pan Xinhui wrote: >>>> strcut __qrwlock has different layout in big endian machine. we need set >>>> the __qrwlock->wmode to NULL, and the address is not &lock->cnts in big >>>> endian machine. >>>> >>>> Do as what read unlock does. we are lucky that the __qrwlock->wmode's >>>> val is _QW_LOCKED. >>> >>> Doesn't this have wider implications for the qrwlocks, for example: >>> >>> while ((cnts & _QW_WMASK) == _QW_LOCKED) { ... } >>> >>> would actually end up looking at the wrong field of the lock? >>> >> I does not clearly understand your idea. :( > > That's because I'm talking rubbish :) Sorry, I completely confused myself. > Locking is bad enough on its own, but add big-endian to the mix and I'm > all done. > >>> Shouldn't we just remove the #ifdef __LITTLE_ENDIAN stuff from __qrwlock, >>> given that all the struct members are u8? >>> >> No. that makes codes complex. for example >> >> struct __qrwlock lock; >> >> WRITE_ONCE(lock->wmode, _QW_WAITING); >> if (atomic_(&lock->cnts) == _QW_WAITING) { >> do_something(); >> } >> >> IF you remove the #ifdef __LITTLE_ENDIAN stuff from __qrwlock. >> codes above obviously will break. And we already have such code. > > I was wondering more along the lines of having one definition of the data > structure, but then defining _QW_* differently depending on endianness > (i.e. add a << 24 when big-endian). That way queued_write_unlock can make sense. And I review all the code, there is not much code to be changed. I will work out one patch based on your idea :) > stay like it is (having an arch override to handle the big-endian case > is incredibly ugly). > I admit that. HOWEVER from the view of performance, having an arch override is acceptable. thanks xinhui > Will >