linux-kernel - Re: [PATCH V2] scsi: core: use blk_mq_requeue_request in __scsi_queue

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Date:   Thu, 1 Mar 2018 09:57:54 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Bart Van Assche <Bart.VanAssche@....com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>
Cc:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH V2] scsi: core: use blk_mq_requeue_request in
 __scsi_queue_insert

Hi Bart

Thanks for your precious time to review this and kindly detailed response.

On 03/01/2018 01:52 AM, Bart Van Assche wrote:
> On Wed, 2018-02-28 at 16:55 +0800, Jianchao Wang wrote:
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index a86df9c..6fa7b0c 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -191,7 +191,8 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, bool unbusy)
>>  	 */
>>  	cmd->result = 0;
>>  	if (q->mq_ops) {
>> -		scsi_mq_requeue_cmd(cmd);
>> +		blk_mq_requeue_request(cmd->request, true);
>> +		put_device(&device->sdev_gendev);
>>  		return;
>>  	}
>>  	spin_lock_irqsave(q->queue_lock, flags);
> 
> Anyone who sees the put_device() call that follows the blk_mq_requeue_request()
> call will wonder why that call occurs there. So I think we need a comment above
> that call that explains where the matching get_device() call is.

Yes, I will add this.

> For the legacy code path, there is a get_device() call in scsi_prep_fn() but no
> put_device() call in scsi_unprep_fn() - the matching put_device() calls occur in
> scsi_end_request() and after blk_requeue_request().
> 
> For scsi-mq however there is a get_device() call in scsi_mq_get_budget() and a
> put_device() call in scsi_mq_put_budget(). So why do we need the put_device()
> calls after blk_mq_requeue_request() and in the mq path for scsi_end_request()?
> 

>From the source code, we know the scsi_mq_get_budget will be invoked every time when we issue a request.
But scsi_mq_put_budget is just in the fail path.

scsi_queue_rq // if any error
  -> scsi_mq_put_budget

blk_mq_dispatch_rq_list // if no driver tags
  -> blk_mq_put_dispatch_budget
    -> scsi_mq_put_budget
blk_mq_do_dispatch_sched/blk_mq_do_dispatch_ctx // if no requests
  -> blk_mq_put_dispatch_budget
    -> scsi_mq_put_budget

So we have to add put_device after  blk_mq_requeue_request() and in scsi_end_request() to match the
scsi_mq_get_budget.

Thanks
Jianchao