Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751778AbdCQKCH (ORCPT ); Fri, 17 Mar 2017 06:02:07 -0400 Received: from mail-pg0-f67.google.com ([74.125.83.67]:36710 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751314AbdCQJ7X (ORCPT ); Fri, 17 Mar 2017 05:59:23 -0400 From: Ming Lei To: Jens Axboe , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, Christoph Hellwig Cc: Yi Zhang , Bart Van Assche , Ming Lei Subject: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler Date: Fri, 17 Mar 2017 17:57:10 +0800 Message-Id: <20170317095711.5819-3-tom.leiming@gmail.com> X-Mailer: git-send-email 2.9.3 In-Reply-To: <20170317095711.5819-1-tom.leiming@gmail.com> References: <20170317095711.5819-1-tom.leiming@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2041 Lines: 57 This patch adds comment on two races related with timeout handler: - requeue from queue busy vs. timeout - rq free & reallocation vs. timeout Both the races themselves and current solution aren't explicit enough, so add comments on them. Cc: Bart Van Assche Signed-off-by: Ming Lei --- block/blk-mq.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/block/blk-mq.c b/block/blk-mq.c index 08a49c69738b..7068779d3bac 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -527,6 +527,15 @@ void blk_mq_start_request(struct request *rq) } EXPORT_SYMBOL(blk_mq_start_request); +/* + * 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); -- 2.9.3