Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753559AbaARKBR (ORCPT ); Sat, 18 Jan 2014 05:01:17 -0500 Received: from e34.co.us.ibm.com ([32.97.110.152]:42544 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752025AbaARKBM (ORCPT ); Sat, 18 Jan 2014 05:01:12 -0500 Date: Sat, 18 Jan 2014 02:01:05 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Linus Torvalds , Matt Turner , Waiman Long , Linux Kernel , Ivan Kokshaysky , Daniel J Blueman , Richard Henderson Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock() Message-ID: <20140118100105.GV10038@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <52D57B60.9020209@twiddle.net> <20140114234443.GY10038@linux.vnet.ibm.com> <20140115023958.GA10038@linux.vnet.ibm.com> <20140115080753.GW31570@twins.programming.kicks-ass.net> <20140115205346.GF10038@linux.vnet.ibm.com> <20140115232134.GM31570@twins.programming.kicks-ass.net> <20140116103659.GO7572@laptop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140116103659.GO7572@laptop.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14011810-1542-0000-0000-0000055759BE Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 16, 2014 at 11:36:59AM +0100, Peter Zijlstra wrote: > On Thu, Jan 16, 2014 at 06:39:23AM +0700, Linus Torvalds wrote: > > On Jan 16, 2014 6:22 AM, "Peter Zijlstra" wrote: > > > > > > So while the primitive is called smp_store_release() the !SMP variant > > > still does: > > > > > > *(volatile __type *) = ptr; > > > > > > which should not compile on any Alpha pre EV56, SMP or no for __type == > > > u8. > > > > I'm not sure where you get that "should not compile" theory from. > > > > I'm pretty sure it will compile just fine. It will just generate the same > > standard read-modify-write sequence (and not using the ldl/stc sequence > > either). Do you have any actual reason to believe it won't, apart from your > > theoretical wishes of how the world should work? > > No, I earlier even said it probably would compile. My usage of 'should' > comes from how we've 'defined' volatile/ACCESS_ONCE in > Documentation/memory-barriers.txt. According to those constraints the > rmw cycle is not proper code. OK, I will bite... Aside from fine-grained code timing, what code could you write to tell the difference between a real one-byte store and an RMW emulating that store? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/