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Date:   Fri, 12 Mar 2021 14:06:41 -0500
From:   Mike Snitzer <snitzer@redhat.com>
To:     Sergei Shtepa <sergei.shtepa@veeam.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Alasdair Kergon <agk@redhat.com>,
        Hannes Reinecke <hare@suse.de>, Jens Axboe <axboe@kernel.dk>,
        dm-devel@redhat.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        pavel.tide@veeam.com
Subject: Re: [PATCH v7 1/3] block: add blk_mq_is_queue_frozen()
Message-ID: <20210312190641.GA2550@redhat.com>
References: <1615563895-28565-1-git-send-email-sergei.shtepa@veeam.com>
 <1615563895-28565-2-git-send-email-sergei.shtepa@veeam.com>
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On Fri, Mar 12 2021 at 10:44am -0500,
Sergei Shtepa <sergei.shtepa@veeam.com> wrote:

> blk_mq_is_queue_frozen() allow to assert that the queue is frozen.
> 
> Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
> ---
>  block/blk-mq.c         | 13 +++++++++++++
>  include/linux/blk-mq.h |  1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index d4d7c1caa439..2f188a865024 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -161,6 +161,19 @@ int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_wait_timeout);
>  
> +bool blk_mq_is_queue_frozen(struct request_queue *q)
> +{
> +	bool frozen;
> +
> +	mutex_lock(&q->mq_freeze_lock);
> +	frozen = percpu_ref_is_dying(&q->q_usage_counter) &&
> +		 percpu_ref_is_zero(&q->q_usage_counter);
> +	mutex_unlock(&q->mq_freeze_lock);
> +
> +	return frozen;
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_is_queue_frozen);
> +

This is returning a frozen state that is immediately stale.  I don't
think any code calling this is providing the guarantees you think it
does due to the racey nature of this state once the mutex is dropped.

>  /*
>   * Guarantee no request is in use, so we can change any data structure of
>   * the queue afterward.
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 2c473c9b8990..6f01971abf7b 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -533,6 +533,7 @@ void blk_freeze_queue_start(struct request_queue *q);
>  void blk_mq_freeze_queue_wait(struct request_queue *q);
>  int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
>  				     unsigned long timeout);
> +bool blk_mq_is_queue_frozen(struct request_queue *q);
>  
>  int blk_mq_map_queues(struct blk_mq_queue_map *qmap);
>  void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues);
> -- 
> 2.20.1
>