From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e39.co.us.ibm.com (e39.co.us.ibm.com [32.97.110.160]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 28EDA1A0014 for ; Sat, 6 Sep 2014 06:19:13 +1000 (EST) Received: from /spool/local by e39.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 5 Sep 2014 14:19:12 -0600 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 9594D3E40030 for ; Fri, 5 Sep 2014 14:19:08 -0600 (MDT) Received: from d03av06.boulder.ibm.com (d03av06.boulder.ibm.com [9.17.195.245]) by b03cxnp08026.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id s85KJ8qt14024820 for ; Fri, 5 Sep 2014 22:19:08 +0200 Received: from d03av06.boulder.ibm.com (loopback [127.0.0.1]) by d03av06.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id s85KNXA7021831 for ; Fri, 5 Sep 2014 14:23:33 -0600 Date: Fri, 5 Sep 2014 13:19:06 -0700 From: "Paul E. McKenney" To: Peter Hurley Subject: Re: bit fields && data tearing Message-ID: <20140905201906.GY5001@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <063D6719AE5E284EB5DD2968C1650D6D17487172@AcuExch.aculab.com> <1409824374.4246.62.camel@pasglop> <5408E458.3@zytor.com> <54090AF4.7060406@hurleysoftware.com> <54091B30.2090509@zytor.com> <20140905081648.GB5281@omega> <20140905180950.GU5001@linux.vnet.ibm.com> <20140905183109.GA5497@linux.vnet.ibm.com> <20140905195234.GT4783@worktop.ger.corp.intel.com> <540A169F.40906@hurleysoftware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <540A169F.40906@hurleysoftware.com> Cc: Jakub Jelinek , "linux-arch@vger.kernel.org" , Tony Luck , "linux-ia64@vger.kernel.org" , Michael Cree , Peter Zijlstra , linux-alpha@vger.kernel.org, Oleg Nesterov , "linux-kernel@vger.kernel.org" , David Laight , Paul Mackerras , "H. Peter Anvin" , "linuxppc-dev@lists.ozlabs.org" , Miroslav Franc , Richard Henderson List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Sep 05, 2014 at 04:01:35PM -0400, Peter Hurley wrote: > On 09/05/2014 03:52 PM, Peter Zijlstra wrote: > > On Fri, Sep 05, 2014 at 11:31:09AM -0700, Paul E. McKenney wrote: > >> compiler: Allow 1- and 2-byte smp_load_acquire() and smp_store_release() > >> > >> CPUs without single-byte and double-byte loads and stores place some > >> "interesting" requirements on concurrent code. For example (adapted > >> from Peter Hurley's test code), suppose we have the following structure: > >> > >> struct foo { > >> spinlock_t lock1; > >> spinlock_t lock2; > >> char a; /* Protected by lock1. */ > >> char b; /* Protected by lock2. */ > >> }; > >> struct foo *foop; > >> > >> Of course, it is common (and good) practice to place data protected > >> by different locks in separate cache lines. However, if the locks are > >> rarely acquired (for example, only in rare error cases), and there are > >> a great many instances of the data structure, then memory footprint can > >> trump false-sharing concerns, so that it can be better to place them in > >> the same cache cache line as above. > >> > >> But if the CPU does not support single-byte loads and stores, a store > >> to foop->a will do a non-atomic read-modify-write operation on foop->b, > >> which will come as a nasty surprise to someone holding foop->lock2. So we > >> now require CPUs to support single-byte and double-byte loads and stores. > >> Therefore, this commit adjusts the definition of __native_word() to allow > >> these sizes to be used by smp_load_acquire() and smp_store_release(). > > > > So does this patch depends on a patch that removes pre EV56 alpha > > support? I'm all for removing that, but I need to see the patch merged > > before we can do this. > > I'm working on that but Alpha's Kconfig is not quite straightforward. > > > ... and I'm wondering if I should _remove_ pre-EV56 configurations or > move the default choice and produce a warning about unsupported Alpha > CPUs instead? I suspect that either would work, given that the Alpha community is pretty close-knit. Just setting the appropriate flag to make the compiler generate one-byte and two-byte loads and stores would probably suffice. ;-) Thanx, Paul > Regards, > Peter Hurley > > [ How does one do a red popup in kbuild? > The 'comment' approach is too subtle. > ] > > >