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

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Date:   Fri, 2 Mar 2018 10:16:37 +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 and detailed summary.

On 03/02/2018 01:43 AM, Bart Van Assche wrote:
> 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.

Supply some details here:
scsi_mq_get_budget before calling .queuecommand get_device and increase device_busy.
scsi_queue_rq increases target_busy and host_busy.

> * 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>


Sincerely
Jianchao