From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 95C0DECDFAA for ; Sat, 14 Jul 2018 12:49:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AB9320890 for ; Sat, 14 Jul 2018 12:49:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2AB9320890 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=codewreck.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727199AbeGNNGc (ORCPT ); Sat, 14 Jul 2018 09:06:32 -0400 Received: from nautica.notk.org ([91.121.71.147]:47410 "EHLO nautica.notk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726447AbeGNNGb (ORCPT ); Sat, 14 Jul 2018 09:06:31 -0400 Received: by nautica.notk.org (Postfix, from userid 1001) id 9C66DC009; Sat, 14 Jul 2018 14:47:30 +0200 (CEST) Date: Sat, 14 Jul 2018 14:47:15 +0200 From: Dominique Martinet To: jiangyiwen Cc: Andrew Morton , Eric Van Hensbergen , Ron Minnich , Latchesar Ionkov , Linux Kernel Mailing List , v9fs-developer@lists.sourceforge.net Subject: Re: [V9fs-developer] [PATCH] net/9p: Fix a deadlock case in the virtio transport Message-ID: <20180714124715.GA16134@nautica> References: <5B49B8CF.40709@huawei.com> <20180714090502.GA16186@nautica> <5B49DAA5.3020600@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <5B49DAA5.3020600@huawei.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org jiangyiwen wrote on Sat, Jul 14, 2018: > On 2018/7/14 17:05, Dominique Martinet wrote: > > jiangyiwen wrote on Sat, Jul 14, 2018: > >> When client has multiple threads that issue io requests all the > >> time, and the server has a very good performance, it may cause > >> cpu is running in the irq context for a long time because it can > >> check virtqueue has buf in the *while* loop. > >> > >> So we should keep chan->lock in the whole loop. > > > > Hmm, this is generally bad practice to hold a spin lock for long. > > In general, spin locks are meant to protect data, not code. > > > > I'd want some numbers to decide on this one, even if I think this > > particular case is safe (e.g. this cannot dead-lock) > > > > Actually, the loop will not hold a spin lock for long, because other > threads will not issue new requests in this case. In addition, > virtio-blk or virtio-scsi also use this solution, I guess it may also > encounter this problem before. Fair enough. If you do have some numbers to give though (throughput and/or iops before/after) I'd still be really curious. > >> chan->ring_bufs_avail = 1; > >> - spin_unlock_irqrestore(&chan->lock, flags); > >> /* Wakeup if anyone waiting for VirtIO ring space. */ > >> wake_up(chan->vc_wq); > > > > In particular, the wake up here echoes to wait events that will > > immediately try to grab the lock, and will needlessly spin on it until > > this thread is done. > > If we do go this way I'd want setting chan->ring_bufs_avail to be done > > just before unlocking and the wakeup to be done just after unlocking out > > of the loop iff we processed at least one iteration here. > > I can move the wakeup operation after the unlocking. Like what I said > above, I think this loop will not execute for long. Please do, you listed virtio_blk as doing this and they have the same kind of pattern with a req_done bool and only restarting stopped queues if they processed something -- Dominique