Re: [patch]blk-mq: blk_mq_tag_to_rq should handle flush request

[Ming Lei]

On Thu, Jun 5, 2014 at 12:09 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 06/04/2014 10:00 AM, Ming Lei wrote:
>> On Wed, Jun 4, 2014 at 11:48 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>> On 06/04/2014 09:43 AM, Ming Lei wrote:
>>>> On Wed, Jun 4, 2014 at 11:31 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>>>>> On Wed, Jun 04, 2014 at 09:02:19AM -0600, Jens Axboe wrote:
>>>>>>> scsi_mq_find_tag only gets the scsi host, which may have multiple
>>>>>>> queues.  When called from scsi_find_tag we actually have a scsi device,
>>>>>>> so that's not an issue, but when called from scsi_host_find_tag the
>>>>>>> driver only provides the host.
>>>>>>
>>>>>> Only solution I see right now is to have the flush_rq in the shared
>>>>>> tags, but that would potentially be a regression for multiple
>>>>>> devices and heavy flush uses cases. I'll see if I can come up with
>>>>>> something better, or maybe Shaohua has an idea.
>>>>>
>>>>> What about something like the following (untest, uncompiled, maybe
>>>>> pseudo-code):
>>>>>
>>>>> struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>>>>> {
>>>>>         struct request *rq = tags->rqs[tag];
>>>>>
>>>>>         if ((rq->cmd_flags & REQ_FLUSH_SEQ) && rq->q->flush_rq->tag == tag)
>>>>>                 return rq->q->flush_rq;
>>>>>         return rq;
>>>>> }
>>>>
>>>> Looks we thought it together, :-)
>>>>
>>>> Also maybe the flush_rq->tag need to be cleared in flush_end_io().
>>>
>>> It clears the command flag, so that should be enough.
>>
>> Only the flush_rq's command flag is cleared, and its parent request
>> flag isn't cleared.
>
> Good point. Care to send in a patch? We can just clear it to -1U, at
> least in blk-mq that's defined as an invalid tag.

Attachment patch should be enough.

Thanks,
-- 
Ming Lei
diff --git a/block/blk-flush.c b/block/blk-flush.c
index ff87c66..8ffee4b 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -225,7 +225,7 @@ static void flush_end_io(struct request *flush_rq, int error)
 
 	if (q->mq_ops) {
 		spin_lock_irqsave(&q->mq_flush_lock, flags);
-		q->flush_rq->cmd_flags = 0;
+		q->flush_rq->tag = -1;
 	}
 
 	running = &q->flush_queue[q->flush_running_idx];