Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752883AbbGIOnl (ORCPT ); Thu, 9 Jul 2015 10:43:41 -0400 Received: from mail-ig0-f173.google.com ([209.85.213.173]:35470 "EHLO mail-ig0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751229AbbGIOne (ORCPT ); Thu, 9 Jul 2015 10:43:34 -0400 MIME-Version: 1.0 In-Reply-To: References: <1435847397-724-1-git-send-email-akinobu.mita@gmail.com> <1435847397-724-2-git-send-email-akinobu.mita@gmail.com> Date: Thu, 9 Jul 2015 22:43:34 +0800 Message-ID: Subject: Re: [PATCH v2 1/6] blk-mq: fix sysfs registration/unregistration race From: Ming Lei To: Akinobu Mita Cc: Linux Kernel Mailing List , Jens Axboe 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: 6921 Lines: 193 On Thu, Jul 9, 2015 at 10:25 PM, Akinobu Mita wrote: > Hi Ming, > > 2015-07-08 20:48 GMT+09:00 Ming Lei : >> On Thu, Jul 2, 2015 at 10:29 PM, Akinobu Mita wrote: >>> There is a race between cpu hotplug handling and adding/deleting >>> gendisk for blk-mq, where both are trying to register and unregister >>> the same sysfs entries. >>> >>> CPU hotplug handling for blk-mq (blk_mq_queue_reinit) does unregister >>> and register sysfs entries of hctx for all request queues in >>> all_q_list. On the other hand, these entries for a request queue may >>> have already been unregistered or in the middle of registration when >>> CPU hotplug event occurs. Because those sysfs entries are registered >>> by blk_mq_register_disk() after the request queue is added to >>> all_q_list, and similarily the request queue is deleted from >>> all_q_list after those are unregistered by blk_mq_unregister_disk(). >>> >>> So we need proper mutual exclusion for those registration and >>> unregistration. >> >> But that may not be enough, and you miss another point in which >> blk_mq_sysfs_register() has to be called after the remapping is built, >> so the introduced lock should have been held in blk_mq_queue_reinit(). > > OK. > > Although q->mq_sysfs_lock is held in blk_mq_sysfs_register() by > this patch, it should be held in blk_mq_queue_reinit() as you > suggested. Since hctx->nr_ctx is reset temporarily in > blk_mq_map_swqueue() while cpu hotplug callback is called, so we > should avoid blk_mq_register_disk() seeing the temporary > hctx->nr_ctx value (i.e. zero) at the same time by holding > q->mq_sysfs_lock in blk_mq_queue_reinit(). > >>> Signed-off-by: Akinobu Mita >>> Cc: Jens Axboe >>> Cc: Ming Lei >>> --- >>> block/blk-core.c | 1 + >>> block/blk-mq-sysfs.c | 44 ++++++++++++++++++++++++++++++++++---------- >>> include/linux/blkdev.h | 3 +++ >>> 3 files changed, 38 insertions(+), 10 deletions(-) >>> >>> diff --git a/block/blk-core.c b/block/blk-core.c >>> index 82819e6..bbf67cd 100644 >>> --- a/block/blk-core.c >>> +++ b/block/blk-core.c >>> @@ -687,6 +687,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id) >>> __set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags); >>> >>> init_waitqueue_head(&q->mq_freeze_wq); >>> + mutex_init(&q->mq_sysfs_lock); >> >> The above should be put into blk_mq_init_allocated_queue(). > > OK, make sense. > >>> >>> if (blkcg_init_queue(q)) >>> goto fail_bdi; >>> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c >>> index b79685e..79a3e8d 100644 >>> --- a/block/blk-mq-sysfs.c >>> +++ b/block/blk-mq-sysfs.c >>> @@ -332,13 +332,15 @@ static void blk_mq_unregister_hctx(struct blk_mq_hw_ctx *hctx) >>> struct blk_mq_ctx *ctx; >>> int i; >>> >>> - if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP)) >>> + if (!(hctx->flags & BLK_MQ_F_SYSFS_UP)) >>> return; >> >> It isn't needed to introduce above change. >> >>> >>> hctx_for_each_ctx(hctx, ctx, i) >>> kobject_del(&ctx->kobj); >>> >>> kobject_del(&hctx->kobj); >>> + >>> + hctx->flags &= ~BLK_MQ_F_SYSFS_UP; >> >> I guess you misunderstand the flag, which just means the syfs stuff >> for the hcts has been setup, and you needn't clear it. > > I've changed the meaning of the flag which should have been > mentioned and also introduced q->mq_sysfs_init_done. So I admit > that the code has become much complicated. > > But after consideration, I think we can only need q->mq_sysfs_init_done > and BLK_MQ_F_SYSFS_UP can be removed. Please see below. > >>> } >>> >>> static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx) >>> @@ -347,7 +349,7 @@ static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx) >>> struct blk_mq_ctx *ctx; >>> int i, ret; >>> >>> - if (!hctx->nr_ctx || !(hctx->flags & BLK_MQ_F_SYSFS_UP)) >>> + if (!hctx->nr_ctx) >>> return 0; >>> >>> ret = kobject_add(&hctx->kobj, &q->mq_kobj, "%u", hctx->queue_num); >>> @@ -357,10 +359,12 @@ static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx) >>> hctx_for_each_ctx(hctx, ctx, i) { >>> ret = kobject_add(&ctx->kobj, &hctx->kobj, "cpu%u", ctx->cpu); >>> if (ret) >>> - break; >>> + return ret; >>> } >>> >>> - return ret; >>> + hctx->flags |= BLK_MQ_F_SYSFS_UP; >> >> Same with above. >> >>> + >>> + return 0; >>> } >>> >>> void blk_mq_unregister_disk(struct gendisk *disk) >>> @@ -370,6 +374,8 @@ void blk_mq_unregister_disk(struct gendisk *disk) >>> struct blk_mq_ctx *ctx; >>> int i, j; >>> >>> + mutex_lock(&q->mq_sysfs_lock); >>> + >>> queue_for_each_hw_ctx(q, hctx, i) { >>> blk_mq_unregister_hctx(hctx); >>> >>> @@ -384,6 +390,9 @@ void blk_mq_unregister_disk(struct gendisk *disk) >>> kobject_put(&q->mq_kobj); >>> >>> kobject_put(&disk_to_dev(disk)->kobj); >>> + >>> + q->mq_sysfs_init_done = false; >> >> The flag isn't needed, but you should hold q->mq_sysfs_lock >> inside blk_mq_queue_reinit(). > > The reason for q->mq_sysfs_init_done is to avoid the following > scenario. > > While the request queue is added to 'all_q_list' (*), > blk_mq_queue_reinit() can be called for the queue anytime by CPU > hotplug callback. But blk_mq_sysfs_unregister (-) and > blk_mq_sysfs_register (+) in blk_mq_queue_reinit must not be > called before blk_mq_register_disk (++) is finished and after > blk_mq_unregister_disk (--). Because '/sys/block/nullb*/mq/' is > not exists. Yes. > > BLK_MQ_F_SYSFS_UP flag in hctx->flags fixes this issue partially now. > In order to fix it completely, we just need q->mq_sysfs_init_done > instead of per hctx flag with appropriate locking. One flag should be OK, but keeping both just makes things complicated, and looks mq_sysfs_init_done is better. > > null_add_dev > --> blk_mq_init_queue > --> blk_mq_init_allocated_queue > --> add to 'all_q_list' (*) > --> add_disk > --> blk_register_queue > --> blk_mq_register_disk (++) > > null_del_dev > --> del_gendisk > --> blk_unregister_queue > --> blk_mq_unregister_disk (--) > --> blk_cleanup_queue > --> blk_mq_free_queue > --> del from 'all_q_list' (*) > > blk_mq_queue_reinit > --> blk_mq_sysfs_unregister (-) > --> blk_mq_sysfs_register (+) -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/