Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753528AbbLNU3O (ORCPT ); Mon, 14 Dec 2015 15:29:14 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:44219 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753211AbbLNU3M (ORCPT ); Mon, 14 Dec 2015 15:29:12 -0500 Date: Mon, 14 Dec 2015 21:28:55 +0100 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Will Deacon , Andrew Pinski , Davidlohr Bueso , Thomas Gleixner , Ingo Molnar , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" , david.daney@cavium.com Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) Message-ID: <20151214202855.GX6357@twins.programming.kicks-ass.net> References: <20151211084133.GE6356@twins.programming.kicks-ass.net> <20151211120419.GD18828@arm.com> <20151211121319.GK6356@twins.programming.kicks-ass.net> <20151211121759.GE18828@arm.com> <20151211122647.GM6356@twins.programming.kicks-ass.net> <20151211133313.GG18828@arm.com> <20151211134803.GP6356@twins.programming.kicks-ass.net> <20151211223540.GA22277@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151211223540.GA22277@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2002 Lines: 46 On Fri, Dec 11, 2015 at 02:35:40PM -0800, Paul E. McKenney wrote: > On Fri, Dec 11, 2015 at 02:48:03PM +0100, Peter Zijlstra wrote: > > On Fri, Dec 11, 2015 at 01:33:14PM +0000, Will Deacon wrote: > > > On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote: > > > > > > While we're there, the acquire in osq_wait_next() seems somewhat ill > > > > documented too. > > > > > > > > I _think_ we need ACQUIRE semantics there because we want to strictly > > > > order the lock-unqueue A,B,C steps and we get that with: > > > > > > > > A: SC > > > > B: ACQ > > > > C: Relaxed > > > > > > > > Similarly for unlock we want the WRITE_ONCE to happen after > > > > osq_wait_next, but in that case we can even rely on the control > > > > dependency there. > > > > > > Even for the lock-unqueue case, isn't B->C ordered by a control dependency > > > because C consists only of stores? > > > > Hmm, indeed. So we could go fully relaxed on it I suppose, since the > > same is true for the unlock site. > > I am probably missing quite a bit on this thread, but don't x86 MMIO > accesses to frame buffers need to interact with something more heavyweight > than an x86 release store or acquire load in order to remain confined > to the resulting critical section? So on x86 there really isn't a problem because every atomic op (and there's plenty here) will be a full barrier. That is, even if you were to replace everything with _relaxed() ops, it would still work as 'expected' on x86. ppc/arm64 will crash and burn, but that's another story. But the important point here was that osq_wait_next() is never relied upon to provide either the ACQUIRE semantics for osq_lock() not the RELEASE semantics for osq_unlock(). Those are provided by other ops. -- 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/