From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency Date: Mon, 7 Jan 2019 11:02:36 -0800 Message-ID: <20190107190236.GF1215@linux.ibm.com> References: <20190102205715.14054-1-mst@redhat.com> <20190102205715.14054-4-mst@redhat.com> <86023cbe-d1ae-a0d6-7b75-26556f1a0c1f@redhat.com> <20190106231756-mutt-send-email-mst@kernel.org> <20190107094610.GA2861@worktop.programming.kicks-ass.net> <20190107082223-mutt-send-email-mst@kernel.org> Reply-To: paulmck@linux.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Peter Zijlstra , Jason Wang , linux-kernel@vger.kernel.org, Alan Stern , Andrea Parri , Will Deacon , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig , linux-arch@vger.kernel.org, netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, Jonathan Corbet , Richard Henderson , Ivan Kokshaysky , Matt Turner , Arnd Bergmann , To: "Michael S. Tsirkin" Return-path: Content-Disposition: inline In-Reply-To: <20190107082223-mutt-send-email-mst@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Mon, Jan 07, 2019 at 08:36:36AM -0500, Michael S. Tsirkin wrote: > On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote: > > On Sun, Jan 06, 2019 at 11:23:07PM -0500, Michael S. Tsirkin wrote: > > > On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote: > > > > On 2019/1/3 上午4:57, Michael S. Tsirkin wrote: > > > > > > > +#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \ > > > > > + !defined(ARCH_NEEDS_READ_BARRIER_DEPENDS) > > > > > + > > > > > +#define dependent_ptr_mb(ptr, val) ({ \ > > > > > + long dependent_ptr_mb_val = (long)(val); \ > > > > > + long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val; \ > > > > > + \ > > > > > + BUILD_BUG_ON(sizeof(val) > sizeof(long)); \ > > > > > + OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val); \ > > > > > + (typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val); \ > > > > > +}) > > > > > + > > > > > +#else > > > > > + > > > > > +#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); }) > > > > > > > > > > > > So for the example of patch 4, we'd better fall back to rmb() or need a > > > > dependent_ptr_rmb()? > > > > > > > > Thanks > > > > > > You mean for strongly ordered architectures like Intel? > > > Yes, maybe it makes sense to have dependent_ptr_smp_rmb, > > > dependent_ptr_dma_rmb and dependent_ptr_virt_rmb. > > > > > > mb variant is unused right now so I'll remove it. > > > > How about naming the thing: dependent_ptr() ? That is without any (r)mb > > implications at all. The address dependency is strictly weaker than an > > rmb in that it will only order the two loads in qestion and not, like > > rmb, any prior to any later load. > > So I'm fine with this as it's enough for virtio, but I would like to point out two things: > > 1. E.g. on x86 both SMP and DMA variants can be NOPs but > the madatory one can't, so assuming we do not want > it to be stronger than rmp then either we want > smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr() > or we just will specify that dependent_ptr() works for > both DMA and SMP. > > 2. Down the road, someone might want to order a store after a load. > Address dependency does that for us too. Assuming we make > dependent_ptr a NOP on x86, we will want an mb variant > which isn't a NOP on x86. Will we want to rename > dependent_ptr to dependent_ptr_rmb at that point? But x86 preserves store-after-load orderings anyway, and even Alpha respects ordering from loads to dependent stores. So what am I missing here? Thanx, Paul