From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Date: Wed, 11 Oct 2017 09:47:48 -0700 Message-ID: <20171011164748.GK3521@linux.vnet.ibm.com> References: <20171011122217.GD11106@arm.com> <20171010155042.GD3521@linux.vnet.ibm.com> <1507594969-8347-12-git-send-email-paulmck@linux.vnet.ibm.com> <20171010001951.GA6476@linux.vnet.ibm.com> <8079.1507628146@warthog.procyon.org.uk> <26455.1507724399@warthog.procyon.org.uk> <6309.1507735045@warthog.procyon.org.uk> <20171011155948.GE3521@linux.vnet.ibm.com> <20171011161220.zqkdhynxerrcmvdd@hirez.programming.kicks-ass.net> <20171011162412.o6lmjiag7spwabge@hirez.programming.kicks-ass.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:38940 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751659AbdJKQrz (ORCPT ); Wed, 11 Oct 2017 12:47:55 -0400 Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9BGhnkD124123 for ; Wed, 11 Oct 2017 12:47:54 -0400 Received: from e18.ny.us.ibm.com (e18.ny.us.ibm.com [129.33.205.208]) by mx0b-001b2d01.pphosted.com with ESMTP id 2dhpgs1wp7-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 11 Oct 2017 12:47:54 -0400 Received: from localhost by e18.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Oct 2017 12:47:54 -0400 Content-Disposition: inline In-Reply-To: <20171011162412.o6lmjiag7spwabge@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: David Howells , Will Deacon , linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, Jonathan Corbet , Alexander Kuleshov , dvyukov@google.com On Wed, Oct 11, 2017 at 06:24:12PM +0200, Peter Zijlstra wrote: > On Wed, Oct 11, 2017 at 06:12:20PM +0200, Peter Zijlstra wrote: > > On Wed, Oct 11, 2017 at 08:59:48AM -0700, Paul E. McKenney wrote: > > > On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote: > > > > I will, however, quibble with the appropriateness of the name READ_ONCE()... > > > > I still think it's not sufficiently obvious that this is a barrier and the > > > > barrier is after. Maybe READ_AND_BARRIER()? > > > > > > Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but > > > indicated that he might consider it if it helped code-analysis tools. > > > Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE() > > > for dependencies would help. Me, I would suggest READ_ONCE_DEP(), but > > > let's figure out if the bikeshed needs to be painted before arguing over > > > the color. ;-) > > > > Count me one vote for the READ_ONCE() name. This is about dependent > > reads, which are nothing special on anything except Alpha. > > > > We want to remove the exception/specialness from the memory model; and > > therefore have to fix up all primitives that could possibly be used for > > these reads to unconditionally issue the barrier (on Alpha). The > > alternative is: rm -rf arch/alpha. > > > > Adding something like READ_ONCE_DEP() does not rid us of the idea that > > dependent reads are special and thus defeats the purpose, we might as > > well retain lockless_dereference(). > > > > Now; any user of dependent reads must use READ_ONCE() in any case, to > > avoid load tearing and reloads. So using READ_ONCE() for the dependent > > reads is not extra or additional (note we'll also have to add the > > barrier to all our relaxed and release atomics and anything else that > > implies READ_ONCE and doesn't already imply smp_mb() after). > > Add the per-cpu ops to that list, they imply READ_ONCE(). Consider for > example this example: > > > for_each_possible_cpu(cpu) > smp_store_release(per_cpu_ptr(&foo, cpu), obj); > > -vs- > > obj = this_cpu_read(foo); > if (obj->ponies) > fart_rainbow(obj); Interesting. Do we currently have any dependencies headed by this_cpu_read()? Thanx, Paul