linux-kernel - Re: USB enumeration post-resume NOT persistent yet "persist" --> swapped devices nodes --> root partition reference broken

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Date:	Thu, 19 Jul 2012 19:30:57 +0200
From:	Andreas Mohr <andi@...as.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: USB enumeration post-resume NOT persistent yet "persist" -->
 swapped devices nodes --> root partition reference broken

Hi,

On Thu, Jul 19, 2012 at 11:11:50AM -0400, Alan Stern wrote:
> On Thu, 19 Jul 2012, Andreas Mohr wrote:
> 
> > Hi,
> > 
> > Yesterday I was surprised to see that with *another* external USB disk
> > happening to be connected before boot,
> > the system booted with root partition device sdb1 assigned rather than sda1.
> > Not thinking much, I then proceeded putting the system into suspend,
> 
> Do you mean "suspend" or "hibernate"?

Doh - S2R. I don't do persistent hibernate here (writing some 1GB of data
to flash-based storage each time possibly isn't all too healthy anyway).

> Can you reproduce the problem?

Will retry soonish.

> > http://lists.linux-foundation.org/pipermail/linux-pm/2009-November/023101.html
> > Netbook Acer Aspire One A110L.
> > Running 3.5.0-rc7+ here (yes ma'am, bleeding edge tester :).
> > Was the first time to attempt resume with an additional device remaining
> > connected, IIRC - that -rc7 thing likely doesn't play much of a role here.
> > A bit hesitant to (dis-)prove the bug's "regression flag" with another version
> > since random possibly succeeding I/O accesses to incompatible devices
> > are not necessarily my thing (or is this safe to attempt again? Any more
> > specific session info one would need?).
> 
> Well, the dmesg log would help.  If you still think the USB layer is at 
> fault then you should enable CONFIG_USB_DEBUG.

Maybe I can get this successfully off the machine next time,
by pre-caching required binaries prior to initiating a non-working resume.

> > So, again, possibly USB persistence is bug-broken?
> 
> You don't have any good evidence to suggest that.  None of the
> information you provided indicates that any USB device nodes (such as
> /dev/bus/usb/001/002) got mixed up.  All you know is that the
> block-layer device nodes (such as /dev/sda2) got changed.

OK - so you're trying in vain to tell dense me that I'm supposed to
take note of the *non-changing* (i.e., correctly "persistent")
USB device ID scheme rather than the roguely changing device nodes.
To which I say that unfortunately I don't have a pre/post comparison
at this moment yet.

> Furthermore, if USB persist were broken then the symptoms would be 
> different.  Instead of starting with a root partition at sdb1 and then 
> finding it at sda1, you would have found it gone completely and there 
> would be _new_ devices labelled sdc and sdd.

Ah, yeah - I tend to know *this* other effect, too...

> Alan Stern

Thanks a ton for your reply!
Now I know that there's a tendency to better look on the other side
(block device layer etc.) and analyze things there,
once it's established that USB topology ID numbering in fact did persist.

Andreas Mohr
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