From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e34.co.us.ibm.com (e34.co.us.ibm.com [32.97.110.152]) (using TLSv1 with cipher CAMELLIA256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 9BCC71A115E for ; Wed, 27 Jan 2016 09:00:02 +1100 (AEDT) Received: from localhost by e34.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 26 Jan 2016 15:00:00 -0700 Received: from b03cxnp08026.gho.boulder.ibm.com (b03cxnp08026.gho.boulder.ibm.com [9.17.130.18]) by d03dlp02.boulder.ibm.com (Postfix) with ESMTP id 841D23E40048 for ; Tue, 26 Jan 2016 14:59:57 -0700 (MST) Received: from d03av05.boulder.ibm.com (d03av05.boulder.ibm.com [9.17.195.85]) by b03cxnp08026.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u0QLxvIn25297134 for ; Tue, 26 Jan 2016 14:59:57 -0700 Received: from d03av05.boulder.ibm.com (localhost [127.0.0.1]) by d03av05.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u0QLxZHH008663 for ; Tue, 26 Jan 2016 14:59:56 -0700 Date: Tue, 26 Jan 2016 11:44:10 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Leonid Yegoshin , Will Deacon , "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , Russell King - ARM Linux , virtualization@lists.linux-foundation.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Joe Perches , David Miller , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net, adi-buildroot-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, xen-devel@lists.xenproject.org, Ralf Baechle , Ingo Molnar , ddaney.cavm@gmail.com, james.hogan@imgtec.com, Michael Ellerman Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-ID: <20160126194410.GQ4503@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20160113104516.GE25458@arm.com> <56969F4B.7070001@imgtec.com> <20160113204844.GV6357@twins.programming.kicks-ass.net> <5696BA6E.4070508@imgtec.com> <20160114120445.GB15828@arm.com> <56980145.5030901@imgtec.com> <20160114204827.GE3818@linux.vnet.ibm.com> <56981212.7050301@imgtec.com> <20160114222046.GH3818@linux.vnet.ibm.com> <20160126102402.GE6357@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160126102402.GE6357@twins.programming.kicks-ass.net> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Jan 26, 2016 at 11:24:02AM +0100, Peter Zijlstra wrote: > On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote: > > On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote: > > > On 01/14/2016 12:48 PM, Paul E. McKenney wrote: > > > > > > > >So SYNC_RMB is intended to implement smp_rmb(), correct? > > > Yes. > > > > > > > >You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > > >smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > > > > > If smp_read_barrier_depends() is used to separate not only two reads > > > but read pointer and WRITE basing on that pointer (example below) - > > > yes. I just doesn't see any example of this in famous > > > Documentation/memory-barriers.txt and had no chance to know what you > > > use it in this way too. > > > > Well, Documentation/memory-barriers.txt was intended as a guide for Linux > > kernel hackers, and not for hardware architects. > > Yeah, this goes under the header: memory-barriers.txt is _NOT_ a > specification (I seem to keep repeating this). > > > ------------------------------------------------------------------------ > > > > commit 955720966e216b00613fcf60188d507c103f0e80 > > Author: Paul E. McKenney > > Date: Thu Jan 14 14:17:04 2016 -0800 > > > > documentation: Subsequent writes ordered by rcu_dereference() > > > > The current memory-barriers.txt does not address the possibility of > > a write to a dereferenced pointer. This should be rare, > > How are these rare? Isn't: > > rcu_read_lock() > obj = rcu_dereference(ptr); > if (!atomic_inc_not_zero(&obj->ref)) > obj = NULL; > rcu_read_unlock(); > > a _very_ common thing to do? It is, but it provides its own barriers, so does not need to rely on dependency ordering. Thanx, Paul