Re: [PATCH v2 1/6] blk-mq: fix sysfs registration/unregistration race

[Ming Lei]

On Thu, Jul 2, 2015 at 10:29 PM, Akinobu Mita <akinobu.mita@xxxxxxxxx> 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().

>
> Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Ming Lei <tom.leiming@xxxxxxxxx>
> ---
>  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().

>
>         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.

>  }
>
>  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().

> +       mutex_unlock(&q->mq_sysfs_lock);
>  }
>
>  static void blk_mq_sysfs_init(struct request_queue *q)
> @@ -414,27 +423,30 @@ int blk_mq_register_disk(struct gendisk *disk)
>         struct blk_mq_hw_ctx *hctx;
>         int ret, i;
>
> +       mutex_lock(&q->mq_sysfs_lock);
> +
>         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;

Same with above.

>                 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:
> +       mutex_unlock(&q->mq_sysfs_lock);
>
> -       return 0;
> +       return ret;
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_register_disk);
>
> @@ -443,8 +455,14 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
>         struct blk_mq_hw_ctx *hctx;
>         int i;
>
> +       mutex_lock(&q->mq_sysfs_lock);
> +       if (!q->mq_sysfs_init_done)
> +               goto out;
> +
>         queue_for_each_hw_ctx(q, hctx, i)
>                 blk_mq_unregister_hctx(hctx);
> +out:
> +       mutex_unlock(&q->mq_sysfs_lock);
>  }
>
>  int blk_mq_sysfs_register(struct request_queue *q)
> @@ -452,11 +470,17 @@ int blk_mq_sysfs_register(struct request_queue *q)
>         struct blk_mq_hw_ctx *hctx;
>         int i, ret = 0;
>
> +       mutex_lock(&q->mq_sysfs_lock);
> +       if (!q->mq_sysfs_init_done)
> +               goto out;

The above check needn't.

> +
>         queue_for_each_hw_ctx(q, hctx, i) {
>                 ret = blk_mq_register_hctx(hctx);
>                 if (ret)
>                         break;
>         }
> +out:
> +       mutex_unlock(&q->mq_sysfs_lock);
>
>         return ret;
>  }
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index d4068c1..c56f5a6 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -462,6 +462,9 @@ struct request_queue {
>
>         struct blk_mq_tag_set   *tag_set;
>         struct list_head        tag_set_list;
> +
> +       struct mutex            mq_sysfs_lock;
> +       bool                    mq_sysfs_init_done;

Same with above.

>  };
>
>  #define QUEUE_FLAG_QUEUED      1       /* uses generic tag queueing */
> --
> 1.9.1
>



-- 
Ming Lei
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