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

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Message-ID: <1519840355.2777.8.camel@wdc.com>
Date:   Wed, 28 Feb 2018 17:52:39 +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 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.

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()?

Thanks,

Bart.