Re: [PATCH V2] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert

jianchao.wang
2018-03-01T01:57:54Z

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