Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754663AbdDMQiS (ORCPT ); Thu, 13 Apr 2017 12:38:18 -0400 Received: from merlin.infradead.org ([205.233.59.134]:53588 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753915AbdDMQiQ (ORCPT ); Thu, 13 Apr 2017 12:38:16 -0400 Date: Thu, 13 Apr 2017 18:37:57 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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 Message-ID: <20170413163757.wwhttkpm3v7emz33@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413162651.GD3956@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1164 Lines: 25 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. 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..