Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751294AbdCQSXt (ORCPT ); Fri, 17 Mar 2017 14:23:49 -0400 Received: from mail-vk0-f53.google.com ([209.85.213.53]:36067 "EHLO mail-vk0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751098AbdCQSXr (ORCPT ); Fri, 17 Mar 2017 14:23:47 -0400 MIME-Version: 1.0 In-Reply-To: <1489772332.2826.6.camel@sandisk.com> References: <20170317095711.5819-1-tom.leiming@gmail.com> <20170317095711.5819-3-tom.leiming@gmail.com> <1489772332.2826.6.camel@sandisk.com> From: Ming Lei Date: Sat, 18 Mar 2017 02:23:25 +0800 Message-ID: Subject: Re: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler To: Bart Van Assche Cc: "linux-kernel@vger.kernel.org" , "hch@infradead.org" , "linux-block@vger.kernel.org" , "axboe@fb.com" , "yizhan@redhat.com" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2568 Lines: 57 On Sat, Mar 18, 2017 at 1:39 AM, Bart Van Assche wrote: > 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 = rq->q; >> @@ -700,6 +709,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx, >> if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) >> return; >> >> + /* >> + * 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, First, this explains how we deal with the race of reuse vs. timeout, and 1/3 fixes another race or rq corruption. Did you see anywhere I mentioned STARTED flag in above comment? In case of 1/3, the rq to be dispatched can be destroyed simply by the blk_mq_end_request() from timeout. Or even it can survive, the same rq can be allocated into another I/O path, and this situation is different with reuse vs. timeout. And I can't see any help from the comment for explaining 1/3's issue, can you? Maybe I need to mention rq corruption in 1/3 explicitly. Secondly introducing this comment to 1/3 just causes unnecessary backporting burden, as we have to make it into -stable. > please consider squashing this patch into patch 1/3. So please do not consider that. Thanks, Ming Lei