linux-kernel - Re: scsi_set_medium

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Date:   Tue, 30 Oct 2018 10:08:13 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Zengtao (B)" <prime.zeng@...ilicon.com>
cc:     "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "usb-storage@...ts.one-eyed-alien.net" 
        <usb-storage@...ts.one-eyed-alien.net>
Subject: Re: scsi_set_medium_removal timeout issue 

On Tue, 30 Oct 2018, Zengtao (B) wrote:

> Hi 
> 
> I have recently met a scsi_set_medium_removal timeout issue, and it's related
>  to both SCSI and USB MASS storage. 
> Since i am not an expert in either scsi or usb mass storage, i am writing to report
> the issue and ask for a solution from you guys.
> 
> My test scenario is as follow:
> 1.Linux HOST-----Linux mass storage gadget(the back store is a flash partition).
> 2.Host mount the device.
> 3.Host writes some data to the Mass storage device.
> 4.Host Unmount the device.
> Both Linux kernels(Host and Device) are Linux 4.9.  
> Some has reported the same issue a long time ago, but it remains there.
> https://www.spinics.net/lists/linux-usb/msg53739.html
> 
> For the issue itself, there is my observation:
> In the step 4, the Host will issue an PREVENT_ALLOW_MEDIUM_REMOVAL scsi command.
> and and timeout happens due to the device 's very slow fsg_lun_fsync_sub.

Something is wrong here.  Before sending PREVENT-ALLOW MEDIUM 
REMOVAL, the host should issue SYNCHRONIZE CACHE.  This will force 
fsg_lun_fsync_sub to run, and the host should allow a long timeout for 
this command.  Then when PREVENT-ALLOW MEDIUM REMOVAL is sent, nothing 
will need to be flushed.

Alan Stern

> I found there are two methods to workaround the issue: 
> 1. Change the timeout value of host scsi command PREVENT_ALLOW_MEDIUM_REMOVAL to
> to about 60 seconds from 10 seconds.
> 2. Remove the fsg_lun_fsync_sub in the device's Mass storage gadget driver.
> 
> Thanks
> 
> Regards
> zentao