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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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