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

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Message-ID: <1519926225.2675.6.camel@wdc.com>
Date:   Thu, 1 Mar 2018 17:43:46 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...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

On Thu, 2018-03-01 at 09:57 +0800, jianchao.wang wrote:
> 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.

Hello Jianchao,

Yes, the block layer core guarantees that scsi_mq_get_budget() will be called
before scsi_queue_rq(). I think the full picture is as follows:
* Before scsi_queue_rq() calls .queuecommand(), get_device() is called for the
  SCSI device and the device, target and host busy counters are incremented.
* If the SCSI core decides to requeue a command, scsi_queue_insert() causes
  __scsi_queue_insert() to call scsi_device_unbusy(). That last function
  decreases the device, target and host busy counters but not put_device(sdev).
  Hence the need for a separate put_device() call after requeuing.

It's unfortunate that the SCSI core became so asymmetric. Anyway, since I am
now convinced that this patch is correct, feel free to add:

Reviewed-by: Bart Van Assche <bart.vanassche@....com>