Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754770AbbLKWfm (ORCPT ); Fri, 11 Dec 2015 17:35:42 -0500 Received: from e36.co.us.ibm.com ([32.97.110.154]:49971 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754230AbbLKWfl (ORCPT ); Fri, 11 Dec 2015 17:35:41 -0500 X-IBM-Helo: d03dlp02.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Fri, 11 Dec 2015 14:35:40 -0800 From: "Paul E. McKenney" To: Peter Zijlstra 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: <20151211223540.GA22277@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151211134803.GP6356@twins.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: 15121122-0021-0000-0000-0000153600A9 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1409 Lines: 36 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? 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/