Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751132AbdCOXl4 (ORCPT ); Wed, 15 Mar 2017 19:41:56 -0400 Received: from mail-pg0-f42.google.com ([74.125.83.42]:33074 "EHLO mail-pg0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751020AbdCOXle (ORCPT ); Wed, 15 Mar 2017 19:41:34 -0400 Date: Thu, 16 Mar 2017 07:41:28 +0800 From: Ming Lei To: Bart Van Assche Cc: "linux-kernel@vger.kernel.org" , "hch@infradead.org" , "linux-block@vger.kernel.org" , "yizhan@redhat.com" , "axboe@fb.com" , "stable@vger.kernel.org" Subject: Re: [PATCH 1/2] blk-mq: don't complete un-started request in timeout handler Message-ID: <20170315234110.GA19908@ming.t460p> References: <1489064578-17305-1-git-send-email-tom.leiming@gmail.com> <1489064578-17305-3-git-send-email-tom.leiming@gmail.com> <1489536441.2676.21.camel@sandisk.com> <20170315121851.GA15807@ming.t460p> <1489613676.2660.8.camel@sandisk.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1489613676.2660.8.camel@sandisk.com> User-Agent: Mutt/1.8.0 (2017-02-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1843 Lines: 46 On Wed, Mar 15, 2017 at 09:34:50PM +0000, Bart Van Assche wrote: > On Wed, 2017-03-15 at 20:18 +0800, Ming Lei wrote: > > On Wed, Mar 15, 2017 at 12:07:37AM +0000, Bart Van Assche wrote: > > > Both the old and the new check look racy to me. The REQ_ATOM_STARTED bit can > > > be changed concurrently by blk_mq_start_request(), __blk_mq_finish_request() > > > > blk_mq_start_request() and __blk_mq_finish_request() won't be run concurrently. > > > > From view of __blk_mq_finish_request(): > > > > - if it is run from merge queue io path(blk_mq_merge_queue_io()), > > blk_mq_start_request() can't be run at all, and the COMPLETE flag > > is kept as previous value(zero) > > > > - if it is run from normal complete path, COMPLETE flag is cleared > > before the req/tag is released to tag set. > > > > so there isn't race in blk_mq_start_request() vs. __blk_mq_finish_request() > > wrt. timeout. > > > > > or __blk_mq_requeue_request(). Another issue with this function is that the > > > > __blk_mq_requeue_request() can be run from two pathes: > > > > - dispatch failure, in which case the req/tag isn't released to tag set > > > > - IO completion path, in which COMPLETE flag is cleared before requeue. > > > > so I can't see races with timeout in case of start rq vs. requeue rq. > > > > > request passed to this function can be reinitialized concurrently. > > Hello Ming, > > You misinterpret what I wrote. I was referring to manipulation of > REQ_ATOM_STARTED from different contexts and not to what you explained. This patch addresses one race between timeout and pre-queuing I/O in block layer (before .queue_rq), please focus on this patch. And that is definitely correct. Also I am happy to discuss this kind of races, but maybe we can do that in another thread if you can describe the issue clearly. -- Ming