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Message-ID: <003101ce846d$1a74ffa0$4f5efee0$@lge.com>
Date: Fri, 19 Jul 2013 19:45:36 +0900
From: "Gioh Kim" <gioh.kim@....com>
To: "'Alan Stern'" <stern@...land.harvard.edu>,
"'Ming Lei'" <tom.leiming@...il.com>
Cc: <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
"'Mark Salter'" <msalter@...hat.com>, <namhyung.kim@....com>,
"'Minchan Kim'" <minchan.kim@....com>,
"'Chanho Min'" <chanho.min@....com>,
"'Jong-Sung Kim'" <neidhard.kim@....com>,
"'linux-arm-kernel'" <linux-arm-kernel@...ts.infradead.org>,
"HyoJun Im" <hyojun.im@....com>
Subject: RE: [PATCH] [RFC] EHCI: add to memory barrier to updating hw_next
Thanks a lot for your replay.
> -----Original Message-----
> From: Alan Stern [mailto:stern@...land.harvard.edu]
> Sent: Thursday, July 18, 2013 11:09 PM
> To: Ming Lei
> Cc: Gioh Kim; linux-usb@...r.kernel.org; linux-kernel@...r.kernel.org;
> Mark Salter; namhyung.kim@....com; Minchan Kim; Chanho Min; Jong-Sung Kim;
> linux-arm-kernel
> Subject: Re: [PATCH] [RFC] EHCI: add to memory barrier to updating hw_next
>
> On Thu, 18 Jul 2013, Ming Lei wrote:
>
> > > I guess that HC could have a use-after-free problem like following
> situation.
> > >
> > > 1. A qtd which is not at the queue head should be removed in
> qh_completions().
> > > 2. The last->hw_next become be pointing at the next qtd but the
> hw_next value is delayed in write-buffer.
> > > 3. The qtd is removed in the list.
> > > 4. The qtd is freed into DMA pool and re-allocated for another urb.
> > > 5. HC try to process last->hw_next and it is pointing re-allocated
qtd.
> > >
> > > What do you think about it? Is it possible?
> >
> > I understand it might not be possible because: when 'stopped' is set,
> > that said the HC might not advance the queue. But I don't understand
> > why 'last->hw_next' is patched here under 'stopped' situation.
>
> It should not be possible. When "stopped" is set, the QH gets unlinked
> and relinked before it can start up again. Relinking involves some memory
> barriers, so the qTD will not be accessed again by the HC.
>
> last->hw_next gets patched because the qTD might belong to some URB in
> the middle of the queue that is being unlinked. The URBs before it and
> after it will still be active, so the queue link has to be updated.
>
You're right. I misunderstand those codes. Please forget about it.
> > Even the 'stopped' case may be seldom triggered, do you know under
> > which condition the stopped is triggered in your problem?(stall, short
> > read or others)
>
> I was going to ask the same question. This particular piece of code gets
> executed _only_ when an URB is unlinked. Not during any other kind of
> error.
I've got the problem when I listened to the mp3 file of USB HDD.
I checked the urb data when the problem occurred, the last-status value of
urb was EINPROGRESS and
urb->unlinked was ECONNRESET.
I think the 'stopped' case was occurred by the reset of USB port.
The block device driver did reset USB port because there is no return from
USB device.
If I made block device driver could not reset USB port, the EHCI driver
codes were not executed.
Finally the halt of HC makes 'stopped' case.
I think halt of the HC might be caused that store-buffer delays command for
HC.
When I applied the patch from https://lkml.org/lkml/2011/8/31/344 and added
a mb() into hw_next updating
to remove delay of store-buffer, My platform works well.
Can the store-buffer delay halt HC? Is it possible?
IMHO, if the qTD list is broken the HC think there is no qTD to send.
So I added mb() at hw_next update code.
>
> Alan Stern
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