Re: [PATCH] Prevent race condition between USB authorisation and USB discovery events

[Alan Stern]

On Mon, 17 Dec 2018, Andrew Worsley wrote:

> A sysfs driven USB authorisation change can trigger a usb_set_configuration
> while a hub_event worker thread is running. This can result in a USB device
> being disabled just after it was configured and bringing down all the
> devices and impacting hardware and user processes that were established on
> top of this these interfaces. In some cases the USB disable never completed
> and the whole system hung.

Can you be more specific about this?  Disabling a USB device shouldn't
cause these kinds of problems, regardless of whether or not the device
was just configured.

> At my work I had an occasional hang due to this race condition. Roughly 1
> in 50 boots had the race occurrence and 1 in 4 of those resulted in a hang.
> This patch fixed the problem and I had no problems (spurious disables
> or hangs) in 750+ boots.

usb_authorize_device, usb_deauthorize_device, and hub_event all acquire 
the device mutex.  Why should adding another mutex make any difference?

In fact there's an actual disadvantage: Making hub_event acquire a
global mutex will prevent us from handling multiple hubs concurrently.  
Although we don't do this now, we might want to in the future.

Alan Stern