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 13:34:50 -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, 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?

Alan Stern

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