From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:49588 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750998AbdE1LLN (ORCPT ); Sun, 28 May 2017 07:11:13 -0400 Date: Sun, 28 May 2017 19:11:06 +0800 From: Ming Lei To: Bart Van Assche Cc: "hch@infradead.org" , "linux-block@vger.kernel.org" , "axboe@fb.com" Subject: Re: [PATCH v2 0/8] blk-mq: fix & improve queue quiescing Message-ID: <20170528111105.GD6488@ming.t460p> References: <20170527142126.26079-1-ming.lei@redhat.com> <1495920721.13651.0.camel@sandisk.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1495920721.13651.0.camel@sandisk.com> Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Sat, May 27, 2017 at 09:32:02PM +0000, Bart Van Assche wrote: > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote: > > There are some issues in current blk_mq_quiesce_queue(): > > > > - in case of direct issue or BLK_MQ_S_START_ON_RUN, dispatch won't > > be prevented after blk_mq_quiesce_queue() is returned. > > - in theory, new RCU read-side critical sections may begin while > > synchronize_rcu() was waiting, and end after returning from > > synchronize_rcu(), then dispatch still may be run after > > synchronize_rcu() returns > > Hello Ming, Hello Bart, > > I disagree with the second part of your statement. blk_mq_quiesce_queue() You can find the similar description from line 158 to line 180. of Documentation/RCU/whatisRCU.txt. For example, there may be schedule preempt or irq handling between checking stopped flag(false) and the RCU read-side critical sections, and synchronize_*rcu() may return before running this RCU read-side critical sections. That is why we have to move the check into RCU read-side critical sections. > stops all hardware queues before calling synchronize_*rcu(). > blk_mq_sched_dispatch_requests() is called with an RCU lock held and checks > whether a hctx has been stopped before calling .queue_rq(). That is > sufficient to prevent blk_mq_try_issue_directly() to trigger a .queue_rq() > call. Our case should be very similar with example "6. ANALOGY WITH READER-WRITER LOCKING" in Documentation/RCU/whatisRCU.txt. Thanks, Ming