From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends() Date: Tue, 5 Dec 2017 14:02:10 -0800 Message-ID: <20171205220210.GV7829@linux.vnet.ibm.com> References: <20171205183946.GP3165@worktop.lehotels.local> <20171205204928-mutt-send-email-mst@kernel.org> <20171205191733.GQ3165@worktop.lehotels.local> <20171205212053-mutt-send-email-mst@kernel.org> <20171205193339.GP7829@linux.vnet.ibm.com> <20171205215020-mutt-send-email-mst@kernel.org> <20171205200801.GR7829@linux.vnet.ibm.com> <20171205222857-mutt-send-email-mst@kernel.org> <20171205213644.GU7829@linux.vnet.ibm.com> <20171205234308-mutt-send-email-mst@kernel.org> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org To: "Michael S. Tsirkin" Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:36998 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752500AbdLEWCT (ORCPT ); Tue, 5 Dec 2017 17:02:19 -0500 Received: from pps.filterd (m0098413.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vB5Lx31U145771 for ; Tue, 5 Dec 2017 17:02:18 -0500 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0b-001b2d01.pphosted.com with ESMTP id 2ep0m691m4-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 05 Dec 2017 17:02:18 -0500 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 5 Dec 2017 17:02:16 -0500 Content-Disposition: inline In-Reply-To: <20171205234308-mutt-send-email-mst@kernel.org> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Dec 05, 2017 at 11:43:41PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 01:36:44PM -0800, Paul E. McKenney wrote: > > On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > > > On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote: > > > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > > > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote: > > > > > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > > > > > > > > [ . . . ] > > > > > > > > > > > and this barrier is no longer paired with anything until > > > > > > > you realize there's a dependency barrier within READ_ONCE. > > > > > > > > > > > > > > Barrier pairing was a useful tool to check code validity, > > > > > > > maybe there are other, better tools now. > > > > > > > > > > > > There are quite a few people who say that smp_store_release() is > > > > > > easier for the tools to analyze than is smp_wmb(). My experience with > > > > > > smp_read_barrier_depends() and rcu_dereference() leads me to believe > > > > > > that they are correct. > > > > > > > > > > OK, but smp_store_release is still not paired with anything since we > > > > > rely on READ_ONCE to include the implicit dpendendency barrier. > > > > > > > > Why wouldn't you consider the smp_store_release() to be paired with > > > > the new improved READ_ONCE()? > > > > > > READ_ONCE is really all over the place (some code literally replaced all > > > memory accesses with READ/WRITE ONCE). > > > > > > And I also prefer smp_wmb as it seems to be cheaper on ARM. > > > > > > Would an API like WRITE_POINTER()/smp_store_pointer make sense, > > > and READ_POINTER for symmetry? > > > > What we do in some code is to comment the pairings, allowing the other > > side of the pairing to be easily located. Would that work for you? > > Yes, that's exactly what I did for now. Very good, thank you! Thanx, Paul