Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752083AbbGIOZl (ORCPT ); Thu, 9 Jul 2015 10:25:41 -0400 Received: from mail-vn0-f52.google.com ([209.85.216.52]:38779 "EHLO mail-vn0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751430AbbGIOZe (ORCPT ); Thu, 9 Jul 2015 10:25:34 -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 23:25:32 +0900 Message-ID: Subject: Re: [PATCH v2 1/6] blk-mq: fix sysfs registration/unregistration race From: Akinobu Mita To: Ming Lei 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: 6487 Lines: 180 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. 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. 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 (+) -- 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/