From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:32872 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727286AbeHaUOj (ORCPT ); Fri, 31 Aug 2018 16:14:39 -0400 Date: Fri, 31 Aug 2018 17:06:40 +0100 From: Will Deacon Subject: Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Message-ID: <20180831160640.GG30626@arm.com> References: <20180831091641.GA3634@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Alan Stern Cc: Andrea Parri , "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com Message-ID: <20180831160640.ZZpqQDWI5SrUUNjzNBygGCgi8sbdLxr5nTE4dVnRQ_o@z> On Fri, Aug 31, 2018 at 10:52:54AM -0400, Alan Stern wrote: > On Fri, 31 Aug 2018, Andrea Parri wrote: > > On Thu, Aug 30, 2018 at 05:31:32PM -0400, Alan Stern wrote: > > > On Thu, 30 Aug 2018, Andrea Parri wrote: > > > > (Remark: ordinary release/acquire are building blocks for code such as > > > > qspinlock, (q)rwlock, mutex, rwsem, ... and what else??). > > > > > > But are these building blocks used the same way for all architectures? > > > > The more, the better! (because then we have the LKMM tools) > > > > We already discussed the "fast path" example: the fast paths of the > > above all resemble: > > > > *_lock(s): atomic_cmpxchg_acquire(&s->val, UNLOCKED_VAL, LOCKED_VAL) ... > > *_unlock(s): ... atomic_set_release(&s->val, UNLOCKED_VAL) > > > > When I read this code, I think "Of course." (unless some arch. has > > messed the implementation of cmpxchg_* up, which can happen...); but > > then I read the subject line of this patch and I think "Wait, what?". > > > > You can argue that this is not generic code, sure; but why on Earth > > would you like to do so?! > > Because the code might not work! On RISC-V, for example, the > implementation of ordinary release/acquire is currently not as strong > as atomic release/acquire. > > Yes, it's true that implementing locks with atomic_cmpxchg_acquire > should be correct on all existing architectures. And Paul has invited > a patch to modify the LKMM accordingly. If you feel that such a change > would be a useful enhancement to the LKMM's applicability, please write > it. Yes, please! That would be the "RmW" discussion which Andrea partially quoted earlier on, so getting that going independently from this patch sounds like a great idea to me. Cheers, Will