Re: [PATCH] USB: fix deadlock in bConfigurationValue attribute method

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On Tue, Apr 17, 2012 at 01:36:28PM -0700, Sarah Sharp wrote:
> On Tue, Apr 17, 2012 at 03:22:39PM -0400, Alan Stern wrote:
> > This patch (as154) fixes a self-deadlock that occurs when userspace
> > writes to the bConfigurationValue sysfs attribute for a hub with
> > children.  The task tries to lock the bandwidth_mutex at a time when
> > it already owns the lock:
> > 
> > 	The attribute's method calls usb_set_configuration(),
> > 	which calls usb_disable_device() with the bandwidth_mutex
> > 	held.
> > 
> > 	usb_disable_device() unregisters the existing interfaces,
> > 	which causes the hub driver to be unbound.
> > 
> > 	The hub_disconnect() routine calls hub_quiesce(), which
> > 	calls usb_disconnect() for each of the hub's children.
> > 
> > 	usb_disconnect() attempts to acquire the bandwidth_mutex
> > 	around a call to usb_disable_device().
> > 
> > The solution is to make usb_disable_device() acquire the mutex for
> > itself instead of requiring the caller to hold it.  Then the mutex can
> > cover only the bandwidth deallocation operation and not the region
> > where the interfaces are unregistered.
> > 
> > This has the potential to change system behavior slightly when a
> > config change races with another config or altsetting change.  Some of
> > the bandwidth released from the old config might get claimed by the
> > other config or altsetting, make it impossible to restore the old
> > config in case of a failure.  But since we don't try to recover from
> > config-change failures anyway, this doesn't matter.
> 
> This should be marked for stable kernels that contain the commit
> fccf4e86200b8f5edd9a65da26f150e32ba79808 "USB: Free bandwidth when
> usb_disable_device is called."
> 
> That commit was marked for stable kernels as old as 2.6.32.

Thanks for the information, I'll add it to the patch.

greg k-h
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