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

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