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Subject: Re: [PATCH] scsi: ufs: make sure scan sequence for multiple hosts
To:     Chanho Park <chanho61.park@samsung.com>, jejb@linux.ibm.com,
        martin.petersen@oracle.com
Cc:     alim.akhtar@samsung.com, avri.altman@wdc.com,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org
References: <CGME20201020070519epcas2p27906d7db7c74e45f2acf8243ec2eae1d@epcas2p2.samsung.com>
 <20201020070516.129273-1-chanho61.park@samsung.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <7fafcc82-2c42-8ef5-14a6-7906b5956363@acm.org>
Date:   Tue, 20 Oct 2020 20:15:13 -0700
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On 10/20/20 12:05 AM, Chanho Park wrote:
> By doing scan as asynchronous way, scsi device scannning can be out of
> order execution. It is no problem if there is a ufs host but the scsi
> device name of each host can be changed according to the scan sequences.
> 
> Ideal Case) host0 scan first
> host0 will be started from /dev/sda
>  -> /dev/sdb (BootLUN0 of host0)
>  -> /dev/sdc (BootLUN1 of host1)
> host1 will be started from /dev/sdd
> 
> This might be an ideal case and we can easily find the host device by
> this mappinng.
> 
> However, Abnormal Case) host1 scan first,
> host1 will be started from /dev/sda and host0 will be followed later.
> 
> To make sure the scan sequences according to the host, we can use a
> bitmap which hosts are scanned and wait until previous hosts are
> finished to scan.

This sounds completely wrong to me. No code should depend on the order in
which LUNs are scanned. Please use the soft links created by udev instead
of serializing LUN scanning.

Thanks,

Bart.