Re: [usb-storage] UAS hangs khubd on USB disconnect

[Sarah Sharp <sarah.a.sharp-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Fri, Dec 13, 2013 at 01:19:36PM -0500, Alan Stern wrote:
> On Fri, 13 Dec 2013, Sarah Sharp wrote:
> 
> > > Given the way things work now, I suspect these warnings are truly 
> > > harmless.  We could simply get rid of the WARN in sysfs_remove_group.
> > > 
> > > The alternative is to call device_del for SCSI targets earlier on, such 
> > > as when their hosts are unregistered.  I don't know how James would 
> > > feel about this approach.  It would be difficult because targets use 
> > > their own reference counts instead of relying on the usual device 
> > > refcounting mechanism.
> > 
> > Thanks for looking into this.  I think just getting rid of the WARN
> > would be sufficient.  Can you make a patch for that?
> 
> Easily.  The downside is that there would no longer be any warning 
> when someone tries to remove a wrong subdirectory by mistake.
> 
> > The patch still won't help with the UAS issues with
> > scsi_init_shared_tag_map though.
> 
> I wasn't clear on the reason for that problem.  Does it also arise from 
> late device_del for scsi_target?  I could try to change the way that 
> works, if anybody (Hans?) would like to test it.

I can certainly test it with my UAS device as well.  I don't know if the
issue arises from the late device_del.  Looking at Hans' stack trace,
the BUG in blk_free_tags gets triggered when the scsi_host is released
before the block_queue release.  So I don't think moving the scsi_target
delete sooner would help?  I really don't know anything about the SCSI
or block layer though.

I can confirm that simply removing the BUG() call in blk_free_tags
allows the partitions on the UAS device to be mounted after it was
hot-removed in the middle of video playback.  Hans, maybe in order to
get an answer to your question[1], you should submit a patch to the
block layer maintainer, Jens Axboe?

Sarah Sharp

[1] http://www.spinics.net/lists/linux-scsi/msg70002.html
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