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

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Date:   Mon, 17 Dec 2018 11:21:55 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrew Worsley <amworsley@...il.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Jon Flatley <jflat@...omium.org>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Bin Liu <b-liu@...com>, Benson Leung <bleung@...omium.org>,
        "open list:USB SUBSYSTEM" <linux-usb@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Prevent race condition between USB authorisation and
 USB discovery events

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