Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754281AbdDMREH (ORCPT ); Thu, 13 Apr 2017 13:04:07 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:40744 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752850AbdDMREG (ORCPT ); Thu, 13 Apr 2017 13:04:06 -0400 Date: Thu, 13 Apr 2017 10:03:49 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, Will Deacon , Boqun Feng , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH tip/core/rcu 02/40] rcu: Make arch select smp_mb__after_unlock_lock() strength Reply-To: paulmck@linux.vnet.ibm.com References: <20170412174003.GA23207@linux.vnet.ibm.com> <1492018825-25634-2-git-send-email-paulmck@linux.vnet.ibm.com> <20170413092418.a2rudzukbgookior@hirez.programming.kicks-ass.net> <20170413162651.GD3956@linux.vnet.ibm.com> <20170413163757.wwhttkpm3v7emz33@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413163757.wwhttkpm3v7emz33@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041317-0056-0000-0000-0000033C150D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006930; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00847071; UDB=6.00417888; IPR=6.00625505; BA=6.00005288; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015033; XFM=3.00000013; UTC=2017-04-13 17:03:54 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041317-0057-0000-0000-0000077220A4 Message-Id: <20170413170349.GK3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-13_11:,, 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=8.0.1-1702020001 definitions=main-1704130143 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1514 Lines: 34 On Thu, Apr 13, 2017 at 06:37:57PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:26:51AM -0700, Paul E. McKenney wrote: > > > ARCH_WEAK_RELEASE_ACQUIRE actually works both ways. > > > > To see this, imagine some strange alternate universe in which the Power > > hardware guys actually did decide to switch PPC to doing RCsc as you > > suggest. There would still be a lot of Power hardware out there that > > still does RCpc. Therefore, powerpc builds that needed to run on old > > Power hardware would select ARCH_WEAK_RELEASE_ACQUIRE, while kernels > > built to run only on the shiny new (but mythical) alternate-universe > > Power hardware would avoid selecting this Kconfig option. > > Ah, but Power software guys could do it today by replacing an LWSYNC > with a SYNC in say arch_spin_unlock(). > > And yes, I know this isn't a popular suggestion, but it would do the > trick. Indeed, there is a fine line between motivating people to move to new hardware on the one hand and terminally annoying existing users on the other. ;-) > Its just that since there's one (PPC) we can sort of pressure them with > the pain of being the only ones to hit all the bugs. But the moment more > appear (and I'm afraid it'll be MIPS, with the excuse that PPC already > does this) it will be ever so much harder to get rid of it. > > Then again, maybe I should just give up and accept the Linux kernel has > RCpc locks.. As usual, I must defer to the powerpc maintainers on this one. Thanx, Paul