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Subject: Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for
 tagset in elevator_exit()
To:     Bart Van Assche <bvanassche@acm.org>, <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>
 <52618092-07ca-ecb5-320f-957af26ab146@acm.org>
From:   John Garry <john.garry@huawei.com>
Message-ID: <3c6cbe11-ac31-9a47-0096-17fbd584b83e@huawei.com>
Date:   Mon, 8 Mar 2021 10:50:55 +0000
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On 06/03/2021 04:32, Bart Van Assche wrote:
> 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);
>>   }

Hi Bart,

> 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?

Firstly - ignoring implementation details for a moment - this patch is 
to ensure that the concept is consistent with your suggestion and 
whether it is sound.

As for nested locking, I can analyze more, but I did assume that we 
don't care about locking-out sysfs intervention during this time. And it 
seems pretty difficult to avoid nesting the locks.

And further to this, I see that 
https://lore.kernel.org/linux-block/3aa5407c-0800-2482-597b-4264781a7eac@grimberg.me/T/#mc3e3175642660578c0ae2a6c32185b1e34ec4b8a 
has a new interface for tagset quiesce, which could make this process 
more efficient.

Please let me know further thoughts.

Thanks,
John