linux-kernel - Re: [PATCH] usb hub: send clear_tt_buffer_complete events when canceling TT clear work

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Date:	Mon, 1 Oct 2012 21:12:34 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb hub: send clear_tt_buffer_complete events when
 canceling TT clear work

Hi Alan,

On Mon, Oct 1, 2012 at 8:48 PM, Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Mon, 1 Oct 2012, Alan Stern wrote:
>
> > On Mon, 1 Oct 2012, Octavian Purdila wrote:
> >
> > > There is a race condition in the USB hub code with regard to handling
> > > TT clear requests that can get the HCD driver in a deadlock. Usually
> > > when an TT clear request is scheduled it will be executed immediately:
> >
> > > However, if a suspend operation is triggered before hub_tt_work is
> > > scheduled, hub_quiesce will cancel the work without notifying the HCD
> > > driver:
> >
> > Ah, this is a very good point.
> >
> > > When the device will resume the EHCI driver will get stuck in
> > > ehci_endpoint_disable waiting for the tt_clearing flag to reset:
> >
> > > This patch notifies the HCD driver via clear_tt_buffer_complete
> > > when canceling TT clear work so that the EHCI driver has a chance to
> > > clear its tt_clearing flag.
> >
> > But I don't like the proposed fix.  What we really need to do is avoid
> > suspending the device until the clear-tt-buffer operation is complete.
> > Can you write a patch to do that instead?
>
> Actually, it would be sufficient to avoid suspending the _hub_ until
> the clear-tt-buffer operation is complete.  In short, hub_quiesce()
> should wait until all pending TT operations have finished.  The
> cancel_work_sync() call should be replaced with flush_work_sync().
>

Good point, i will resend the patch. I will only be able to test the
fix tomorrow though, I don't have access to the setup that reproduces
the issue.

Thanks!
Tavi
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