From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751375AbcFNKkj (ORCPT ); Tue, 14 Jun 2016 06:40:39 -0400 Received: from foss.arm.com ([217.140.101.70]:57819 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750806AbcFNKkh (ORCPT ); Tue, 14 Jun 2016 06:40:37 -0400 Date: Tue, 14 Jun 2016 11:40:33 +0100 From: Will Deacon To: xinhui 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 Message-ID: <20160614104032.GE19407@arm.com> References: <1464862148-5672-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <20160608092244.GA27029@arm.com> <575FA024.7060608@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <575FA024.7060608@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 stay like it is (having an arch override to handle the big-endian case is incredibly ugly). Will