linux-kernel - Re: [PATCH] USB: fix deadlock in HCD code

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Message-ID: <48343764.7020800@csr.com>
Date:	Wed, 21 May 2008 15:53:24 +0100
From:	David Vrabel <david.vrabel@....com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Oliver Neukum <oliver@...kum.org>, Jiri Kosina <jkosina@...e.cz>,
	Greg KH <greg@...ah.com>, linux-usb@...r.kernel.org,
	lchiquitto@...ell.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: fix deadlock in HCD code

Alan Stern wrote:
> On Wed, 21 May 2008, David Vrabel wrote:
> 
>> Alan Stern wrote:
>>> The functions you are worried about (usb_hcd_link_urb_to_ep and 
>>> usb_hcd_unlink_urb_from_ep) are documented as requiring that interrupts 
>>> be disabled by their callers.  This patch isn't needed.
>> This requirement is the only reason the whci-hcd driver disables
>> interrupts.  Removing this requirement would reduce the time that
>> interrupts are disabled in the whci-hcd driver.
> 
> That doesn't sound like a valid approach.  If you don't disable
> interrupts then you aren't protected against an interrupt handler
> submitting an URB and accessing your data structures while whci-hcd is
> in the middle of updating them.

I can't see how urbs can be submitted to whci-hcd from an interrupt
handler (urb callbacks are always called from a workqueue thread) but
they could be submitted from a timer, so you are correct.

David
-- 
David Vrabel, Senior Software Engineer, Drivers
CSR, Churchill House, Cambridge Business Park,  Tel: +44 (0)1223 692562
Cowley Road, Cambridge, CB4 0WZ                 http://www.csr.com/
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