linux-kernel - Re: [PATCH] usb: host: xhci: Replace bus lock with host controller lock

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Date:	Thu, 4 Feb 2016 16:00:51 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Chris Bainbridge <chris.bainbridge@...il.com>
cc:	mathias.nyman@...ux.intel.com, <johan@...nel.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: host: xhci: Replace bus lock with host controller
 lock

On Thu, 4 Feb 2016, Chris Bainbridge wrote:

> The XHCI controller presents two USB buses to the system - one for USB 2
> and one for USB 3. When only one bus is locked there is a race condition
> during hub init that results in errors like:
> 
>  [   13.183701] usb 3-3: device descriptor read/all, error -110

What exactly is the race condition?  Why does locking both buses fix 
it?


> @@ -4312,7 +4312,7 @@ hub_port_init(struct usb_hub *hub, struct usb_device *udev, int port1,
>  	if (oldspeed == USB_SPEED_LOW)
>  		delay = HUB_LONG_RESET_TIME;
>  
> -	mutex_lock(&hdev->bus->usb_address0_mutex);
> +	mutex_lock(&hdev->bus->controller->mutex);
>  
>  	/* Reset the device; full speed may morph to high speed */
>  	/* FIXME a USB 2.0 device may morph into SuperSpeed on reset. */
> @@ -4588,7 +4588,7 @@ fail:
>  		hub_port_disable(hub, port1, 0);
>  		update_devnum(udev, devnum);	/* for disconnect processing */
>  	}
> -	mutex_unlock(&hdev->bus->usb_address0_mutex);
> +	mutex_unlock(&hdev->bus->controller->mutex);
>  	return retval;
>  }

I don't think this is a good idea.  The driver core needs to be able to
access the controller while this function is running.  You can
introduce a new mutex if you want, perhaps in the primary hcd
structure, but don't use bus->controller->mutex.

Alan Stern