From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from esa4.hgst.iphmx.com ([216.71.154.42]:11197 "EHLO esa4.hgst.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751101AbdCQRkR (ORCPT ); Fri, 17 Mar 2017 13:40:17 -0400 From: Bart Van Assche To: "linux-kernel@vger.kernel.org" , "hch@infradead.org" , "linux-block@vger.kernel.org" , "tom.leiming@gmail.com" , "axboe@fb.com" CC: "yizhan@redhat.com" Subject: Re: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler Date: Fri, 17 Mar 2017 17:39:06 +0000 Message-ID: <1489772332.2826.6.camel@sandisk.com> References: <20170317095711.5819-1-tom.leiming@gmail.com> <20170317095711.5819-3-tom.leiming@gmail.com> In-Reply-To: <20170317095711.5819-3-tom.leiming@gmail.com> Content-Type: text/plain; charset="iso-8859-1" MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote: > +/* > + * When we reach here because queue is busy, REQ_ATOM_COMPLETE > + * flag isn't set yet, so there may be race with timeout hanlder, > + * but given rq->deadline is just set in .queue_rq() under > + * this sitation, the race won't be possible in reality because > + * rq->timeout should be set as big enough to cover the window > + * between blk_mq_start_request() called from .queue_rq() and > + * clearing REQ_ATOM_STARTED here. > + */ > static void __blk_mq_requeue_request(struct request *rq) > { > struct request_queue *q =3D rq->q; > @@ -700,6 +709,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ct= x *hctx, > if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) > return; > =20 > + /* > + * The rq being checked may have been freed and reallocated > + * out already here, we avoid this race by checking rq->deadline > + * and REQ_ATOM_COMPLETE flag together: > + * > + * - if rq->deadline is observed as new value because of > + * reusing, the rq won't be timed out because of timing. > + * - if rq->deadline is observed as previous value, > + * REQ_ATOM_COMPLETE flag won't be cleared in reuse path > + * because we put a barrier between setting rq->deadline > + * and clearing the flag in blk_mq_start_request(), so > + * this rq won't be timed out too. > + */ > if (time_after_eq(jiffies, rq->deadline)) { > if (!blk_mark_rq_complete(rq)) > blk_mq_rq_timed_out(rq, reserved); Since this explanation applies to the same race addressed by patch 1/3, please consider squashing this patch into patch 1/3. Thanks, Bart.=