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Date:   Tue, 4 Jul 2023 20:51:10 +0200
From:   Benjamin Block <bblock@linux.ibm.com>
To:     Marc Hartmayer <mhartmay@linux.ibm.com>
Cc:     Yu Kuai <yukuai1@huaweicloud.com>, linux-scsi@vger.kernel.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        yangerkun@huawei.com, hch@lst.de, chaitanyak@nvidia.com,
        shinichiro.kawasaki@wdc.com, dgilbert@interlog.com,
        jejb@linux.ibm.com, martin.petersen@oracle.com, yukuai3@huawei.com,
        axboe@kernel.dk
Subject: Re: [PATCH] scsi/sg: don't grab scsi host module reference
Message-ID: <20230704185110.GA10217@t480-pf1aa2c2>
References: <20230621160111.1433521-1-yukuai1@huaweicloud.com>
 <87lefv622n.fsf@linux.ibm.com>
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On Tue, Jul 04, 2023 at 07:04:00PM +0200, Marc Hartmayer wrote:
> On Thu, Jun 22, 2023 at 12:01 AM +0800, Yu Kuai <yukuai1@huaweicloud.com> wrote:
> > From: Yu Kuai <yukuai3@huawei.com>
> > diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> > index 2433eeef042a..dcb73787c29d 100644
> > --- a/drivers/scsi/sg.c
> > +++ b/drivers/scsi/sg.c
> > @@ -1497,7 +1497,7 @@ sg_add_device(struct device *cl_dev)
> >  	int error;
> >  	unsigned long iflags;
> >  
> > -	error = scsi_device_get(scsidp);
> > +	error = blk_get_queue(scsidp->request_queue);
> >  	if (error)
> >  		return error;
> >  

Might be interesting as well. Marc showed me a `dmesg` snipped earlier
from when the bind fails:

  [   15.441817] scsi host2: scsi_eh_2: sleeping
  [   15.441899] scsi_debug:sdebug_driver_probe: scsi_debug: trim poll_queues to 0. poll_q/nr_hw = (0/1)
  [   15.441907] scsi host2: scsi_debug: version 0191 [20210520]
                   dev_size_mb=8, opts=0x0, submit_queues=1, statistics=0
  [   15.442078] scsi host2: scsi_scan_host_selected: <4294967295:4294967295:18446744073709551615>
  [   15.442267] scsi 2:0:0:0: scsi scan: INQUIRY pass 1 length 36
  [   15.442286] scsi 2:0:0:0: scsi scan: INQUIRY successful with code 0x0
  [   15.442296] scsi 2:0:0:0: scsi scan: INQUIRY pass 2 length 96
  [   15.442308] scsi 2:0:0:0: scsi scan: INQUIRY successful with code 0x0
  [   15.442317] scsi 2:0:0:0: Direct-Access     Linux    scsi_debug       0191 PQ: 0 ANSI: 7
  [   15.442554] scsi 2:0:0:0: Power-on or device reset occurred
  [   15.442560] scsi 2:0:0:0: tag#50 Done: SUCCESS Result: hostbyte=DID_OK driverbyte=DRIVER_OK cmd_age=0s
  [   15.442565] scsi 2:0:0:0: tag#50 CDB: Report supported operation codes a3 0c 01 88 00 00 00 00 00 14 00 00
  [   15.442569] scsi 2:0:0:0: tag#50 Sense Key : Unit Attention [current]
  [   15.442573] scsi 2:0:0:0: tag#50 Add. Sense: Power on occurred

The bind should happend around here somewhere I think.

  [   15.472680] sd 2:0:0:0: scsi scan: Sending REPORT LUNS to (try 0)
  [   15.472703] sd 2:0:0:0: scsi scan: REPORT LUNS successful (try 0) result 0x0
  [   15.472706] sd 2:0:0:0: scsi scan: REPORT LUN scan
  [   15.472709] sd 2:0:0:0: scsi scan: device exists on 2:0:0:0
  [   15.492874] sd 2:0:0:0: [sdi] 16384 512-byte logical blocks: (8.39 MB/8.00 MiB)
  [   15.502853] sd 2:0:0:0: [sdi] Write Protect is off
  [   15.502856] sd 2:0:0:0: [sdi] Mode Sense: 73 00 10 08
  [   15.522819] sd 2:0:0:0: [sdi] Write cache: enabled, read cache: enabled, supports DPO and FUA
  [   15.552773] sd 2:0:0:0: [sdi] Preferred minimum I/O size 512 bytes
  [   15.552776] sd 2:0:0:0: [sdi] Optimal transfer size 524288 bytes
  [   15.575373] sd 2:0:0:0: [sdi] tag#62 Done: SUCCESS Result: hostbyte=DID_OK driverbyte=DRIVER_OK cmd_age=0s
  [   15.575377] sd 2:0:0:0: [sdi] tag#62 CDB: Inquiry 12 01 b9 00 04 00
  [   15.575380] sd 2:0:0:0: [sdi] tag#62 Sense Key : Illegal Request [current]
  [   15.575383] sd 2:0:0:0: [sdi] tag#62 Add. Sense: Invalid field in cdb
  [   15.645749] sd 2:0:0:0: [sdi] Attached SCSI disk

But we don't even see the `sg_alloc: dev=...` message that is logged in
`sg_alloc()`. And between the change above and the call to `sg_alloc()`,
there is only the character device allocation; and if that failed, it
would print an error. So either the bind is never even tried, or the new
`blk_get_queue()` fails to get a reference.
    Which is odd, since the only way that would happen is, if the queue
was marked dying; but we see that the stack is using it for LUN probing
in `sd`.

> This change (bisected) triggers a regression in our KVM on s390x CI. The
> symptom is that a “scsi_debug device” does not bind to the scsi_generic
> driver. On s390x you can reproduce the problem as follows (I have not
> tested on x86):
> 
> With this patch applied:
> 
> $ sudo modprobe scsi_debug

One more thing maybe worth mentioning: in the kernel configuration we
use in the CI we have `sg` built-in. I guess most have it built as
module.

> $ # Get the 'scsi_host,channel,target_number,LUN' tuple for the scsi_debug device
> $ lsscsi |grep scsi_debug |awk '{ print $1 }'
> [0:0:0:0]
> $ sudo stat /sys/bus/scsi/devices/0:0:0:0/scsi_generic
> stat: cannot statx '/sys/bus/scsi/devices/0:0:0:0/scsi_generic': No such file or directory
> 
> 
> Patch reverted:
> 
> $ sudo modprobe scsi_debug
> $ lsscsi |grep scsi_debug |awk '{ print $1 }'
> [0:0:0:0]
> $ sudo stat /sys/bus/scsi/devices/0:0:0:0/scsi_generic
>   File: /sys/bus/scsi/devices/0:0:0:0/scsi_generic
>   Size: 0         	Blocks: 0          IO Block: 4096   directory
> Device: 0,20	Inode: 12155       Links: 3
> …

That's all I got from looking at it earlier, so far.

-- 
Best Regards, Benjamin Block        /        Linux on IBM Z Kernel Development
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