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

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Date:   Thu, 23 Apr 2020 16:49:00 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     John Garry <john.garry@...wei.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     axboe@...nel.dk, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        ming.lei@...hat.com, bvanassche@....org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        esc.storagedev@...rosemi.com, chenxiang66@...ilicon.com,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved
 commands

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@...e.de                               +49 911 74053 688
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