From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3w3nFM6dzSzDq7g for ; Fri, 14 Apr 2017 03:04:07 +1000 (AEST) Received: from pps.filterd (m0098416.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v3DH40OK183659 for ; Thu, 13 Apr 2017 13:04:01 -0400 Received: from e19.ny.us.ibm.com (e19.ny.us.ibm.com [129.33.205.209]) by mx0b-001b2d01.pphosted.com with ESMTP id 29t1rk04w4-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 13 Apr 2017 13:04:00 -0400 Received: from localhost by e19.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 13 Apr 2017 13:03:55 -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 In-Reply-To: <20170413163757.wwhttkpm3v7emz33@hirez.programming.kicks-ass.net> Message-Id: <20170413170349.GK3956@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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