linux-kernel - RE: [PATCH] usb: uas: fix usb subsystem hang after power off hub port

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Date:   Mon, 15 Apr 2019 11:18:30 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Kento.A.Kobayashi@...y.com
cc:     bvanassche@....org, <martin.petersen@...cle.com>,
        <jejb@...ux.ibm.com>, <oneukum@...e.com>,
        <gregkh@...uxfoundation.org>,
        <usb-storage@...ts.one-eyed-alien.net>, <Jacky.Cao@...y.com>,
        <linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        <linux-usb@...r.kernel.org>
Subject: RE: [PATCH] usb: uas: fix usb subsystem hang after power off hub
 port

On Mon, 15 Apr 2019 Kento.A.Kobayashi@...y.com wrote:

> Hi
> 
> >The unbind happens from inside the SCSI EH callback.  If that really is not allowed, we'll need to change it.  Or we can just change it regardless, since the effort required is pretty small.
> >
> >Kento, please try the patch below.  Does it help with your problem?
> 
> Thank you for suggestion about this problem.
> I confirmed your patch fixes this problem.

Good; I will submit it.

> I think you change policy for error handler to not calling unbind, right?

That's right.

> In addition, I have a question about this patch.
> Could you please tell me why it should not be allowed that the unbind is occurred from eh callback?

The SCSI core does not handle unbind properly when it occurs inside an 
error handler callback.  If you want to know more about why the SCSI 
core behaves this way, you should ask the SCSI developers -- I don't 
know the answer.

> This patch will ignore all error which is returned from usb_reset_and_verify_device.
> But my patch will ignore error only being returned ENODEV case.
> I think side effect of your patch is bigger than my patch.

The only other errors that usb_reset_and_verify_device() can return are 
EINVAL and EISDIR.  These error codes occur in three situations:

	The device has already been disconnected;

	The device has no parent hub (it is a root hub);

	The device is currently suspended.

The first situation is just as bad as -ENODEV.  The second cannot
happen for a USB mass storage device.  The third can happen only if an
error occurs when usb_reset_device() tries to carry out a resume, and
that's also just as bad as -ENODEV.

So although the side effects are larger than with your patch, they are 
not any worse.  Furthermore, they handle correctly some situations that 
your patch does not handle.

> So I want to know why the unbind is occurred from eh callback should not be allowed.

Ask the SCSI developers.

Alan Stern