Re: [PATCH 2/2] block: only return started requests from blk_mq_tag_to_rq()

[Jens Axboe <axboe@kernel.dk>]

On 6/19/20 8:09 AM, Hannes Reinecke wrote:
> On 6/19/20 4:01 PM, Hannes Reinecke wrote:
>> blk_mq_tag_to_rq() is used from within the driver to map a tag
>> to a request. As such it should only return requests which are
>> already started (ie passed to the driver); otherwise the driver
>> might trip over requests which it has never seen and random
>> crashes will occur.
>>
>> Signed-off-by: Hannes Reinecke <hare@suse.de>
>> ---
>>   block/blk-mq.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 4f57d27bfa73..f02d18113f9e 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -815,9 +815,13 @@ EXPORT_SYMBOL(blk_mq_delay_kick_requeue_list);
>>   
>>   struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>>   {
>> +	struct request *rq;
>> +
>>   	if (tag < tags->nr_tags) {
>>   		prefetch(tags->rqs[tag]);
>> -		return tags->rqs[tag];
>> +		rq = tags->rqs[tag];
>> +		if (blk_mq_request_started(rq))
>> +			return rq;
>>   	}
>>   
>>   	return NULL;
>>
> This becomes particularly obnoxious for SCSI drivers using 
> scsi_host_find_tag() for cleaning up stale commands (ie drivers like 
> qla4xxx, fnic, and snic).
> All other drivers use it from the completion routine, so one can expect 
> a valid (and started) tag here. So for those it shouldn't matter.
> 
> But still, if there are objections I could look at fixing it within the 
> SCSI stack; although that would most likely mean I'll have to implement 
> the above patch as an additional function.

The helper does exactly what it should, return a request associated
with a tag. Either add the logic to the caller, or provide a new
helper that does what you need. I'd be inclined to just add it to
the caller that needs it.

-- 
Jens Axboe