linux-kernel - Re: [RFC PATCH v3 1/3] blk-mq: Clean up references to old requests when freeing rqs

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Message-ID: <ba6bd7b2-ac24-9e62-795d-d494434fc152@huawei.com>
Date:   Mon, 8 Mar 2021 10:37:51 +0000
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>, <hare@...e.de>,
        <ming.lei@...hat.com>, <axboe@...nel.dk>, <hch@....de>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pragalla@...eaurora.org>, <kashyap.desai@...adcom.com>,
        <yuyufen@...wei.com>
Subject: Re: [RFC PATCH v3 1/3] blk-mq: Clean up references to old requests
 when freeing rqs

On 06/03/2021 18:13, Bart Van Assche wrote:
> On 3/5/21 7:14 AM, John Garry wrote:
>> @@ -2296,10 +2296,14 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
>>   
>>   		for (i = 0; i < tags->nr_tags; i++) {
>>   			struct request *rq = tags->static_rqs[i];
>> +			int j;
>>   
>>   			if (!rq)
>>   				continue;
>>   			set->ops->exit_request(set, rq, hctx_idx);
>> +			/* clean up any references which occur in @ref_tags */
>> +			for (j = 0; ref_tags && j < ref_tags->nr_tags; j++)
>> +				cmpxchg(&ref_tags->rqs[j], rq, 0);
>>   			tags->static_rqs[i] = NULL;
>>   		}
>>   	}

Hi Bart,

> What prevents blk_mq_tagset_busy_iter() from reading hctx->tags[...]
> before the cmpxcg() call and dereferencing it after blk_mq_free_rqs()
> has called __free_pages()?
> 

So there is nothing in this patch to stop that. But it's pretty 
unlikely, as the window is very narrow generally between reading 
hctx->tags[...] and actually dereferencing it. However, something like 
that should be made safe in patch 2/3.

Thanks,
John