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 13:48:05 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Octavian Purdila <octavian.purdila@...el.com>
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

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().

Alan Stern

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