From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757343AbaEPPRN (ORCPT ); Fri, 16 May 2014 11:17:13 -0400 Received: from mail-ie0-f180.google.com ([209.85.223.180]:47688 "EHLO mail-ie0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755745AbaEPPRL (ORCPT ); Fri, 16 May 2014 11:17:11 -0400 Message-ID: <53762BF4.2040701@kernel.dk> Date: Fri, 16 May 2014 09:17:08 -0600 From: Jens Axboe User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Ming Lei CC: Linux Kernel Mailing List , Rusty Russell Subject: Re: [PATCH] virtio_blk: fix race between start and stop queue References: <1400157181-17800-1-git-send-email-tom.leiming@gmail.com> <53762662.2050702@kernel.dk> <5376275F.8030709@kernel.dk> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2014-05-16 09:15, Ming Lei wrote: > On Fri, May 16, 2014 at 10:57 PM, Jens Axboe wrote: >> On 2014-05-16 08:53, Jens Axboe wrote: >>> >>> On 2014-05-15 06:33, Ming Lei wrote: >>>> >>>> When there isn't enough vring descriptor for adding to vq, >>>> blk-mq will be put as stopped state until some of pending >>>> descriptors are completed & freed. >>>> >>>> Unfortunately, the vq's interrupt may come just before >>>> blk-mq's BLK_MQ_S_STOPPED flag is set, so the blk-mq will >>>> still be kept as stopped even though lots of descriptors >>>> are completed and freed in the interrupt handler. The worst >>>> case is that all pending descriptors are freed in the >>>> interrupt handler, and the queue is kept as stopped forever. >>>> >>>> This patch fixes the problem by starting/stopping blk-mq >>>> with holding vq_lock. >>> >>> >>> Why not just use blk_mq_start_hw_queues()? >> >> >> Or, if you want to maintain current heuristics, just move the start and stop >> under the vq_lock. That should prevent the race, as far as I can tell. Not >> sure what that extra queue_stopped would buy you, seems a lot cleaner to >> just maintain this state exclusively in the queue. > > Yes. > > But the flag can avoid to call blk_mq_start_stopped_hw_queues() > unnecessarily, which needn't at most of times. Considered that > the interrupt may happen with very high frequency, I suggest to > introduce the extra flag. virtio-blk just has one queue, so the flag is at least pointless for now. And since the other code stops all of them anyway, I don't see any reason not to just rely on that. -- Jens Axboe