Re: [PATCH v2] watchdog: add driver for StreamLabs USB watchdog device

[Guenter Roeck]

On 04/18/2016 01:32 AM, Oliver Neukum wrote:
> On Mon, 2016-04-18 at 03:53 +0100, Alexey Klimov wrote:
> > This patch creates new driver that supports StreamLabs usb watchdog
> > device. This device plugs into 9-pin usb header and connects to
> > reset pin and reset button on common PC.
> >
> > USB commands used to communicate with device were reverse
> > engineered using usbmon.
>
> Almost. I see only one issue.
>
> > +struct streamlabs_wdt {
> > +	struct watchdog_device wdt_dev;
> > +	struct usb_interface *intf;
> > +
> > +	struct mutex lock;
> > +	u8 buffer[BUFFER_LENGTH];
>
> That is wrong.
>
> > +};
> > +
>
> [..]
>
> > +static int usb_streamlabs_wdt_command(struct watchdog_device *wdt_dev, u16 cmd)
> > +{
> > +	struct streamlabs_wdt *streamlabs_wdt = watchdog_get_drvdata(wdt_dev);
> > +	struct usb_device *usbdev;
> > +	int retval;
> > +	int size;
> > +	unsigned long timeout_msec;
> > +
> > +	int retry_counter = 10;		/* how many times to re-send stop cmd */
> > +
> > +	mutex_lock(&streamlabs_wdt->lock);
> > +
> > +	if (unlikely(!streamlabs_wdt->intf)) {
> > +		mutex_unlock(&streamlabs_wdt->lock);
> > +		return -ENODEV;
> > +	}
> > +
> > +	usbdev = interface_to_usbdev(streamlabs_wdt->intf);
> > +	timeout_msec = wdt_dev->timeout * MSEC_PER_SEC;
> > +
> > +	do {
> > +		usb_streamlabs_wdt_prepare_buf((u16 *) streamlabs_wdt->buffer,
> > +							cmd, timeout_msec);
> > +		/* send command to watchdog */
> > +		retval = usb_interrupt_msg(usbdev, usb_sndintpipe(usbdev, 0x02),
> > +				streamlabs_wdt->buffer, BUFFER_TRANSFER_LENGTH,
>
> Because of this line.
>
> The problem is subtle. Your buffer and your lock share a cacheline.
> On some architecture the cache is not consistent with respect to DMA.
> On them cachelines holding a buffer for DMA need to be flushed to RAM
> and invalidated and you may read from them only after DMA has finished.
>
> Thus you may have prepared a cacheline for DMA but somebody tries taking
> the lock. Then the cacheline with the lock is read from RAM. If that
> happens before you finish the DMA the data resulting from DMA is lost.
>
> The fix is to allocate the buffer with its own allocation. The VM
> subsystem makes sure separate allocation don't share cachelines.

Hi Oliver,

For my own education, would adding ____cacheline_aligned to the buffer variable
declaration solve the problem as well ?

Thanks,
Guenter

> That is the long explanation for what I mean when I say that you violate
> the DMA rules.
>
> 	Regards
> 		Oliver