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From: Oliver Neukum <oliver@neukum.org>
To: Alex Riesen <raa.lkml@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>, Jens Axboe <axboe@kernel.dk>,
        linux-usb@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: USB device cannot be reconnected and khubd "blocked for more than 120 seconds"
Date: Sun, 13 Jan 2013 20:16:11 +0100
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On Sunday 13 January 2013 18:42:49 Alex Riesen wrote:
> On Sun, Jan 13, 2013 at 5:56 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> > On Sun, 13 Jan 2013, Alex Riesen wrote:
> >>
> >> Yes, almost. What about khubd hanging when machine is shutdown?
> >
> > What about it?  I have trouble understanding all the descriptions you
> > have provided so far, because you talk about several different things
> > and change your mind a lot.  Can you provide a single, simple scenario
> > that illustrates this problem?
> 
> 1. Compile a kernel with deadline elevator as module
> 2. Boot into it, make sure the elevator is selected
>   (I used "elevator=deadline" in the kernel command line)
> 3. Insert a FAT formatted mass storage device in an USB2 port
>    Observe "io scheduler deadline registered"
> 4. Pull the stick out, wait a moment, and either shutdown or just
>    and press alt-sysrq-W:

That makes it clear. The elevator probably has scheduled work
which cannot finish waiting on a lock and scsi_remove_host()
wants to flush work.

This is not a USB problem. You need to involve the SCSI people.
khubd just stops working because disconnects are processed
in its context and the removal deadlocks.

	Regards
		Oliver