Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands

From: Hannes Reinecke <hare@suse.de>
To: John Garry <john.garry@huawei.com>,
	Christoph Hellwig <hch@infradead.org>
Cc: axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com,
	ming.lei@redhat.com, bvanassche@acm.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-scsi@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	esc.storagedev@microsemi.com, chenxiang66@hisilicon.com,
	Hannes Reinecke <hare@suse.com>
Subject: Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands
Date: Thu, 23 Apr 2020 16:49:00 +0200	[thread overview]
Message-ID: <a0316b0b-a24c-7d0c-df17-0573593e2a11@suse.de> (raw)
In-Reply-To: <ae3b498b-aea8-dc09-53b8-9e160effc681@huawei.com>

On 4/23/20 4:13 PM, John Garry wrote:
> On 07/04/2020 17:30, Christoph Hellwig wrote:
>> On Tue, Apr 07, 2020 at 04:00:10PM +0200, Hannes Reinecke wrote:
>>> My concern is this:
>>>
>>> struct scsi_device *scsi_get_host_dev(struct Scsi_Host *shost)
>>> {
>>>     [ .. ]
>>>     starget = scsi_alloc_target(&shost->shost_gendev, 0, 
>>> shost->this_id);
>>>     [ .. ]
>>>
>>> and we have typically:
>>>
>>> drivers/scsi/hisi_sas/hisi_sas_v3_hw.c: .this_id                = -1,
>>>
>>> It's _very_ uncommon to have a negative number as the SCSI target 
>>> device; in
>>> fact, it _is_ an unsigned int already.
>>>
>>> But alright, I'll give it a go; let's see what I'll end up with.
>>
>> But this shouldn't be exposed anywhere.  And I prefer that over having
>> magic requests/scsi_cmnd that do not have a valid ->device pointer.
>> .
>>
> 
> (just looking at this again)
> 
> Hi Christoph,
> 
> So how would this look added in scsi_lib.c:
> 
> struct scsi_cmnd *scsi_get_reserved_cmd(struct Scsi_Host *shost)
> {
>      struct scsi_cmnd *scmd;
>      struct request *rq;
>      struct scsi_device *sdev = scsi_get_host_dev(shost);
> 
>      if (!sdev)
>          return NULL;
> 
>      rq = blk_mq_alloc_request(sdev->request_queue,
>                    REQ_OP_DRV_OUT | REQ_NOWAIT,
>                    BLK_MQ_REQ_RESERVED);
>      if (IS_ERR(rq)) // fix tidy-up
>          return NULL;
>      WARN_ON(rq->tag == -1);
>      scmd = blk_mq_rq_to_pdu(rq);
>      scmd->request = rq;
>      scmd->device = sdev;
> 
>      return scmd;
> }
> EXPORT_SYMBOL_GPL(scsi_get_reserved_cmd);
> 
> void scsi_put_reserved_cmd(struct scsi_cmnd *scmd)
> {
>      struct request *rq = blk_mq_rq_from_pdu(scmd);
> 
>      if (blk_mq_rq_is_reserved(rq)) {
>          struct scsi_device *sdev = scmd->device;
>          blk_mq_free_request(rq);
>          scsi_free_host_dev(sdev);
>      }
> }
> EXPORT_SYMBOL_GPL(scsi_put_reserved_cmd);
> 
> Not sure if we want a static scsi_device per host, or alloc and free 
> dynamically.
> 
> (@Hannes, I also have some proper patches for libsas if you want to add it)
> 
Hold your horses.
I'm currently preparing a patchset implementing things by improving the 
current scsi_get_host_dev() etc.

RFC should be ready by the end of the week.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
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