From patchwork Tue Mar 7 20:38:05 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 9609857 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 824216016C for ; Tue, 7 Mar 2017 21:14:23 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 54DB328544 for ; Tue, 7 Mar 2017 21:14:23 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 47F2D28546; Tue, 7 Mar 2017 21:14:23 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 96D8928544 for ; Tue, 7 Mar 2017 21:14:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751579AbdCGVOW (ORCPT ); Tue, 7 Mar 2017 16:14:22 -0500 Received: from mx2.suse.de ([195.135.220.15]:37382 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755817AbdCGVOT (ORCPT ); Tue, 7 Mar 2017 16:14:19 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de X-Amavis-Alert: BAD HEADER SECTION, Duplicate header field: "Cc" Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id D6D6FABCD; Tue, 7 Mar 2017 20:38:14 +0000 (UTC) From: NeilBrown To: Jens Axboe , Jack Wang Date: Wed, 08 Mar 2017 07:38:05 +1100 Cc: LKML , Lars Ellenberg , Kent Overstreet , Pavel Machek , Mike Snitzer , Mikulas Patocka Cc: linux-raid@vger.kernel.org, device-mapper development , linux-block@vger.kernel.org Subject: [PATCH v2] blk: improve order of bio handling in generic_make_request() In-Reply-To: References: <87h93blz6g.fsf@notabene.neil.brown.name> <71562c2c-97f4-9a0a-32ec-30e0702ca575@profitbricks.com> <87lgsjj9w8.fsf@notabene.neil.brown.name> Message-ID: <87r328j00i.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP To avoid recursion on the kernel stack when stacked block devices are in use, generic_make_request() will, when called recursively, queue new requests for later handling. They will be handled when the make_request_fn for the current bio completes. If any bios are submitted by a make_request_fn, these will ultimately be handled seqeuntially. If the handling of one of those generates further requests, they will be added to the end of the queue. This strict first-in-first-out behaviour can lead to deadlocks in various ways, normally because a request might need to wait for a previous request to the same device to complete. This can happen when they share a mempool, and can happen due to interdependencies particular to the device. Both md and dm have examples where this happens. These deadlocks can be erradicated by more selective ordering of bios. Specifically by handling them in depth-first order. That is: when the handling of one bio generates one or more further bios, they are handled immediately after the parent, before any siblings of the parent. That way, when generic_make_request() calls make_request_fn for some particular device, we can be certain that all previously submited requests for that device have been completely handled and are not waiting for anything in the queue of requests maintained in generic_make_request(). An easy way to achieve this would be to use a last-in-first-out stack instead of a queue. However this will change the order of consecutive bios submitted by a make_request_fn, which could have unexpected consequences. Instead we take a slightly more complex approach. A fresh queue is created for each call to a make_request_fn. After it completes, any bios for a different device are placed on the front of the main queue, followed by any bios for the same device, followed by all bios that were already on the queue before the make_request_fn was called. This provides the depth-first approach without reordering bios on the same level. This, by itself, it not enough to remove all deadlocks. It just makes it possible for drivers to take the extra step required themselves. To avoid deadlocks, drivers must never risk waiting for a request after submitting one to generic_make_request. This includes never allocing from a mempool twice in the one call to a make_request_fn. A common pattern in drivers is to call bio_split() in a loop, handling the first part and then looping around to possibly split the next part. Instead, a driver that finds it needs to split a bio should queue (with generic_make_request) the second part, handle the first part, and then return. The new code in generic_make_request will ensure the requests to underlying bios are processed first, then the second bio that was split off. If it splits again, the same process happens. In each case one bio will be completely handled before the next one is attempted. With this is place, it should be possible to disable the punt_bios_to_recover() recovery thread for many block devices, and eventually it may be possible to remove it completely. Ref: http://www.spinics.net/lists/raid/msg54680.html Tested-by: Jinpu Wang Inspired-by: Lars Ellenberg Signed-off-by: NeilBrown --- block/blk-core.c | 25 +++++++++++++++++++++---- 1 file changed, 21 insertions(+), 4 deletions(-) Changes since v1: - merge code improvements from Jack Wang - more edits to changelog comment - add Ref: link. - Add some lists to Cc, that should have been there the first time. diff --git a/block/blk-core.c b/block/blk-core.c index b9e857f4afe8..9520e82aa78c 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -2018,17 +2018,34 @@ blk_qc_t generic_make_request(struct bio *bio) struct request_queue *q = bdev_get_queue(bio->bi_bdev); if (likely(blk_queue_enter(q, false) == 0)) { + struct bio_list hold; + struct bio_list lower, same; + + /* Create a fresh bio_list for all subordinate requests */ + hold = bio_list_on_stack; + bio_list_init(&bio_list_on_stack); ret = q->make_request_fn(q, bio); blk_queue_exit(q); - bio = bio_list_pop(current->bio_list); + /* sort new bios into those for a lower level + * and those for the same level + */ + bio_list_init(&lower); + bio_list_init(&same); + while ((bio = bio_list_pop(&bio_list_on_stack)) != NULL) + if (q == bdev_get_queue(bio->bi_bdev)) + bio_list_add(&same, bio); + else + bio_list_add(&lower, bio); + /* now assemble so we handle the lowest level first */ + bio_list_merge(&bio_list_on_stack, &lower); + bio_list_merge(&bio_list_on_stack, &same); + bio_list_merge(&bio_list_on_stack, &hold); } else { - struct bio *bio_next = bio_list_pop(current->bio_list); - bio_io_error(bio); - bio = bio_next; } + bio = bio_list_pop(current->bio_list); } while (bio); current->bio_list = NULL; /* deactivate */