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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 53A6AECDFB0 for ; Sat, 14 Jul 2018 11:18:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B8DD3208A3 for ; Sat, 14 Jul 2018 11:18:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B8DD3208A3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=huawei.com 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 S1726881AbeGNLb3 (ORCPT ); Sat, 14 Jul 2018 07:31:29 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:45806 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725978AbeGNLb3 (ORCPT ); Sat, 14 Jul 2018 07:31:29 -0400 Received: from DGGEMS408-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id AC88692BE3564; Sat, 14 Jul 2018 19:12:42 +0800 (CST) Received: from [127.0.0.1] (10.177.16.168) by DGGEMS408-HUB.china.huawei.com (10.3.19.208) with Microsoft SMTP Server id 14.3.382.0; Sat, 14 Jul 2018 19:12:38 +0800 Subject: Re: [V9fs-developer] [PATCH] net/9p: Fix a deadlock case in the virtio transport To: Dominique Martinet References: <5B49B8CF.40709@huawei.com> <20180714090502.GA16186@nautica> CC: Andrew Morton , Eric Van Hensbergen , Ron Minnich , Latchesar Ionkov , Linux Kernel Mailing List , From: jiangyiwen Message-ID: <5B49DAA5.3020600@huawei.com> Date: Sat, 14 Jul 2018 19:12:37 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <20180714090502.GA16186@nautica> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.16.168] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. >> Signed-off-by: Yiwen Jiang >> --- >> net/9p/trans_virtio.c | 8 +++----- >> 1 file changed, 3 insertions(+), 5 deletions(-) >> >> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c >> index 05006cb..9b0f5f2 100644 >> --- a/net/9p/trans_virtio.c >> +++ b/net/9p/trans_virtio.c >> @@ -148,20 +148,18 @@ static void req_done(struct virtqueue *vq) >> >> p9_debug(P9_DEBUG_TRANS, ": request done\n"); >> >> + spin_lock_irqsave(&chan->lock, flags); >> while (1) { >> - spin_lock_irqsave(&chan->lock, flags); >> req = virtqueue_get_buf(chan->vq, &len); >> - if (req == NULL) { >> - spin_unlock_irqrestore(&chan->lock, flags); >> + if (req == NULL) >> break; >> - } >> 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. Thanks, Yiwen. > That should also save you precious cpu cycles while under lock :) >