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

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Date:   Thu, 01 Mar 2018 20:43:08 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     "jianchao.w.wang\@oracle.com" <jianchao.w.wang@...cle.com>
Cc:     Bart Van Assche <Bart.VanAssche@....com>,
        "jejb\@linux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen\@oracle.com" <martin.petersen@...cle.com>,
        "linux-scsi\@vger.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch\@lst.de" <hch@....de>
Subject: Re: [PATCH V2] scsi: core: use blk_mq_requeue_request in __scsi_queue_insert


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:

Please add something akin to Bart's explanation as a comment and repost.

Thanks!

-- 
Martin K. Petersen	Oracle Linux Engineering