Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752034AbbKXAA0 (ORCPT ); Mon, 23 Nov 2015 19:00:26 -0500 Received: from quartz.orcorp.ca ([184.70.90.242]:47395 "EHLO quartz.orcorp.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751804AbbKXAAZ (ORCPT ); Mon, 23 Nov 2015 19:00:25 -0500 Date: Mon, 23 Nov 2015 17:00:11 -0700 From: Jason Gunthorpe To: Caitlin Bestler Cc: Bart Van Assche , Christoph Hellwig , "linux-rdma@vger.kernel.org" , "sagig@dev.mellanox.co.il" , "axboe@fb.com" , "linux-scsi@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 2/9] IB: add a proper completion queue abstraction Message-ID: <20151124000011.GA9301@obsidianresearch.com> References: <20151113220636.GA32133@obsidianresearch.com> <20151114071344.GE27738@lst.de> <20151123203712.GB5640@obsidianresearch.com> <56537F59.4080708@sandisk.com> <20151123212822.GE6062@obsidianresearch.com> <56538AFD.9080103@sandisk.com> <20151123221806.GA7152@obsidianresearch.com> <56539421.9050705@sandisk.com> <20151123230659.GA8287@obsidianresearch.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) X-Broken-Reverse-DNS: no host name found for IP address 10.0.0.160 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1791 Lines: 40 On Mon, Nov 23, 2015 at 03:30:42PM -0800, Caitlin Bestler wrote: > The receive completion can be safely assumed to indicate transmit > completion over a reliable connection unless your peer has gone > completely bonkers and is replying to a command that it did not > receive. Perhaps iWarp is different and does specify this ordering but IB does not. The issue with IB is how the ACK protocol is designed. There is not strong ordering between ACKs and data transfers. A HCA can send ACK,DATA and the network could drop the ACK. The recevier side does not know the ACK was lost and goes ahead to process DATA. Since only ACK advances the sendq and DATA advances the recvq it is trivial to get a case where the recvq is advanced with a reply while the sendq continues to wait for the ACK to be resent. Further IB allows ACK coalescing and has no rules for how an ACK is placed. It is entirely valid for a HCA to RECV,REPLY,ACK - for instance. > I actually had a bug in an early iWARP emulation where the simulated > peer, because it was simulated, responded > instantly. The result was a TCP segment that both acked the > transmission *and* contained the reply. The bug was > that the code processed the reception before the transmission ack, > causing the receive completion to be placed > on the completion queue before transmit completion. I don't know if iWARP has the same lax ordering as IB, but certainly, what you describe is legal for IB verbs to do, and our kernel ULPs have to cope with it. Jason -- 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/