Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for
 tagset in elevator_exit()
To:     John Garry <john.garry@huawei.com>, hare@suse.de,
        ming.lei@redhat.com, axboe@kernel.dk, hch@lst.de
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        pragalla@codeaurora.org, kashyap.desai@broadcom.com,
        yuyufen@huawei.com
References: <1614957294-188540-1-git-send-email-john.garry@huawei.com>
 <1614957294-188540-3-git-send-email-john.garry@huawei.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <52618092-07ca-ecb5-320f-957af26ab146@acm.org>
Date:   Fri, 5 Mar 2021 20:32:24 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.1
MIME-Version: 1.0
In-Reply-To: <1614957294-188540-3-git-send-email-john.garry@huawei.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 3/5/21 7:14 AM, John Garry wrote:
> diff --git a/block/blk.h b/block/blk.h
> index 3b53e44b967e..1a948bfd91e4 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -201,10 +201,29 @@ void elv_unregister_queue(struct request_queue *q);
>  static inline void elevator_exit(struct request_queue *q,
>  		struct elevator_queue *e)
>  {
> +	struct blk_mq_tag_set *set = q->tag_set;
> +	struct request_queue *tmp;
> +
>  	lockdep_assert_held(&q->sysfs_lock);
>  
> +	mutex_lock(&set->tag_list_lock);
> +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> +		if (tmp == q)
> +			continue;
> +		blk_mq_freeze_queue(tmp);
> +		blk_mq_quiesce_queue(tmp);
> +	}
> +
>  	blk_mq_sched_free_requests(q);
>  	__elevator_exit(q, e);
> +
> +	list_for_each_entry(tmp, &set->tag_list, tag_set_list) {
> +		if (tmp == q)
> +			continue;
> +		blk_mq_unquiesce_queue(tmp);
> +		blk_mq_unfreeze_queue(tmp);
> +	}
> +	mutex_unlock(&set->tag_list_lock);
>  }

This patch introduces nesting of tag_list_lock inside sysfs_lock. The
latter is per request queue while the former can be shared across
multiple request queues. Has it been analyzed whether this is safe?

Thanks,

Bart.