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Message-ID: <CACVXFVOBSx1rSrPHpgsepctCHCHAbCOJv4ncwBM8H0chPLV3UQ@mail.gmail.com>
Date:	Sun, 19 Jul 2015 19:47:02 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v3 2/7] blk-mq: fix sysfs registration/unregistration race

On Sun, Jul 19, 2015 at 12:28 AM, Akinobu Mita <akinobu.mita@...il.com> 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.
>
> 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 (+)
>
> 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 (++) and after blk_mq_unregister_disk (--)
> is finished.  Because '/sys/block/*/mq/' is not exists.
>
> There has already been BLK_MQ_F_SYSFS_UP flag in hctx->flags which can
> be used to track these sysfs stuff, but it is only fixing this issue
> partially.
>
> In order to fix it completely, we just need per-queue flag instead of
> per-hctx flag with appropriate locking.  So this introduces
> q->mq_sysfs_init_done which is properly protected with all_q_mutex.
>
> Also, we need to ensure that blk_mq_map_swqueue() is called with
> all_q_mutex is held.  Since hctx->nr_ctx is reset temporarily and
> updated in blk_mq_map_swqueue(), so we should avoid
> blk_mq_register_hctx() seeing the temporary hctx->nr_ctx value
> in CPU hotplug handling or adding/deleting gendisk .
>
> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Ming Lei <tom.leiming@...il.com>

Reviewed-by: Ming Lei <tom.leiming@...il.com>

> ---
>  block/blk-mq-sysfs.c   | 30 ++++++++++++++++++++++--------
>  block/blk-mq.c         |  6 +++---
>  include/linux/blk-mq.h |  1 -
>  include/linux/blkdev.h |  2 ++
>  4 files changed, 27 insertions(+), 12 deletions(-)
>
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index b79685e..79096a6 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -332,7 +332,7 @@ 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->nr_ctx)
>                 return;
>
>         hctx_for_each_ctx(hctx, ctx, i)
> @@ -347,7 +347,7 @@ 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);
> @@ -370,6 +370,8 @@ void blk_mq_unregister_disk(struct gendisk *disk)
>         struct blk_mq_ctx *ctx;
>         int i, j;
>
> +       blk_mq_disable_hotplug();
> +
>         queue_for_each_hw_ctx(q, hctx, i) {
>                 blk_mq_unregister_hctx(hctx);
>
> @@ -384,6 +386,9 @@ 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;
> +       blk_mq_enable_hotplug();
>  }
>
>  static void blk_mq_sysfs_init(struct request_queue *q)
> @@ -414,27 +419,30 @@ int blk_mq_register_disk(struct gendisk *disk)
>         struct blk_mq_hw_ctx *hctx;
>         int ret, i;
>
> +       blk_mq_disable_hotplug();
> +
>         blk_mq_sysfs_init(q);
>
>         ret = kobject_add(&q->mq_kobj, kobject_get(&dev->kobj), "%s", "mq");
>         if (ret < 0)
> -               return ret;
> +               goto out;
>
>         kobject_uevent(&q->mq_kobj, KOBJ_ADD);
>
>         queue_for_each_hw_ctx(q, hctx, i) {
> -               hctx->flags |= BLK_MQ_F_SYSFS_UP;
>                 ret = blk_mq_register_hctx(hctx);
>                 if (ret)
>                         break;
>         }
>
> -       if (ret) {
> +       if (ret)
>                 blk_mq_unregister_disk(disk);
> -               return ret;
> -       }
> +       else
> +               q->mq_sysfs_init_done = true;
> +out:
> +       blk_mq_enable_hotplug();
>
> -       return 0;
> +       return ret;
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_register_disk);
>
> @@ -443,6 +451,9 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
>         struct blk_mq_hw_ctx *hctx;
>         int i;
>
> +       if (!q->mq_sysfs_init_done)
> +               return;
> +
>         queue_for_each_hw_ctx(q, hctx, i)
>                 blk_mq_unregister_hctx(hctx);
>  }
> @@ -452,6 +463,9 @@ int blk_mq_sysfs_register(struct request_queue *q)
>         struct blk_mq_hw_ctx *hctx;
>         int i, ret = 0;
>
> +       if (!q->mq_sysfs_init_done)
> +               return ret;
> +
>         queue_for_each_hw_ctx(q, hctx, i) {
>                 ret = blk_mq_register_hctx(hctx);
>                 if (ret)
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f29f766..68921b7 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2045,13 +2045,13 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>                 goto err_hctxs;
>
>         mutex_lock(&all_q_mutex);
> -       list_add_tail(&q->all_q_node, &all_q_list);
> -       mutex_unlock(&all_q_mutex);
>
> +       list_add_tail(&q->all_q_node, &all_q_list);
>         blk_mq_add_queue_tag_set(set, q);
> -
>         blk_mq_map_swqueue(q);
>
> +       mutex_unlock(&all_q_mutex);
> +
>         return q;
>
>  err_hctxs:
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 37d1602..b80ba45 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -145,7 +145,6 @@ enum {
>         BLK_MQ_F_SHOULD_MERGE   = 1 << 0,
>         BLK_MQ_F_TAG_SHARED     = 1 << 1,
>         BLK_MQ_F_SG_MERGE       = 1 << 2,
> -       BLK_MQ_F_SYSFS_UP       = 1 << 3,
>         BLK_MQ_F_DEFER_ISSUE    = 1 << 4,
>         BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
>         BLK_MQ_F_ALLOC_POLICY_BITS = 1,
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index d4068c1..b02c90b 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -462,6 +462,8 @@ struct request_queue {
>
>         struct blk_mq_tag_set   *tag_set;
>         struct list_head        tag_set_list;
> +
> +       bool                    mq_sysfs_init_done;
>  };
>
>  #define QUEUE_FLAG_QUEUED      1       /* uses generic tag queueing */
> --
> 1.9.1
>



-- 
Ming Lei
--
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