From mboxrd@z Thu Jan 1 00:00:00 1970 From: xinhui Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian Date: Wed, 15 Jun 2016 11:47:21 +0800 Message-ID: <5760CFC9.6020006@linux.vnet.ibm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20160614104032.GE19407@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Will Deacon Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, arnd@arndb.de, waiman.long@hp.com, peterz@infradead.org List-Id: linux-arch.vger.kernel.org On 2016=E5=B9=B406=E6=9C=8814=E6=97=A5 18:40, Will Deacon wrote: > On Tue, Jun 14, 2016 at 02:11:48PM +0800, xinhui wrote: >> >> On 2016=E5=B9=B406=E6=9C=8808=E6=97=A5 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 ne= ed set >>>> the __qrwlock->wmode to NULL, and the address is not &lock->cnts i= n big >>>> endian machine. >>>> >>>> Do as what read unlock does. we are lucky that the __qrwlock->wmod= e's >>>> val is _QW_LOCKED. >>> >>> Doesn't this have wider implications for the qrwlocks, for example: >>> >>> while ((cnts & _QW_WMASK) =3D=3D _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 my= self. > 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 __qr= wlock, >>> 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) =3D=3D _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 endiannes= s > (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 cha= nged. I will work out one patch based on your idea :) > stay like it is (having an arch override to handle the big-endian cas= e > is incredibly ugly). > I admit that. HOWEVER from the view of performance, having an arch over= ride is acceptable. thanks xinhui > Will >