Re: [PATCH v3 2/9] watchdog: Introduce hardware maximum timeout in watchdog core

[Guenter Roeck <linux@roeck-us.net>]

Hi Uwe,

On 09/08/2015 03:33 AM, Uwe Kleine-König wrote:
> Hello,
>

>> [...]
>> +static long watchdog_next_keepalive(struct watchdog_device *wdd)
>> +{
>> +	unsigned int hw_timeout_ms = wdd->timeout * 1000;
>> +	unsigned long keepalive_interval;
>> +	unsigned long last_heartbeat;
>> +	unsigned long virt_timeout;
>> +
>> +	virt_timeout = wdd->last_keepalive + msecs_to_jiffies(hw_timeout_ms);
>
> Just looking at this line this is wrong. It just happens to be correct
> here because hw_timeout_ms non-intuitively is set to wdd->timeout * 1000
> which might not reflect what is programmed into the hardware.
>
I don't see where the code is wrong. Sure, the variable name doesn't match
its initial use, but that doesn't make it wrong. I can pick a different variable
name if that helps (any suggested name ?).

> I'd write:
>
> 	virt_timeout = wdd->last_keepalive + msecs_to_jiffies(wdd->timeout * 1000);
>
> ...
>
>> +	if (hw_timeout_ms > wdd->max_hw_timeout_ms)
>> +		hw_timeout_ms = wdd->max_hw_timeout_ms;
>
> 	hw_timeout_ms = min(wdd->timeout * 1000, wdd->max_hw_timeout_ms);
>

The reason for writing the code as is was to avoid the double 'wdd->timeout * 1000'
(and to avoid a line > 80 columns in the first line).

>> [...]
>> @@ -61,26 +143,27 @@ static struct watchdog_device *old_wdd;
>>
>>   static int watchdog_ping(struct watchdog_device *wdd)
>>   {
>> -	int err = 0;
>> +	int err;
>>
>>   	mutex_lock(&wdd->lock);
>> +	wdd->last_keepalive = jiffies;
>> +	err = _watchdog_ping(wdd);
>> +	watchdog_update_worker(wdd, false);
>
> Here the cancel argument could also be true, right? That's because after
> a ping (that doesn't modify the timeout) the result of
> watchdog_need_worker doesn't change and so either the worker isn't
> running + stopping it again doesn't hurt, or the timer is running and so
> it's not tried to be stopped.
>
Could, but it isn't necessary.

>> +	mutex_unlock(&wdd->lock);
>>
>> -	if (test_bit(WDOG_UNREGISTERED, &wdd->status)) {
>> -		err = -ENODEV;
>> -		goto out_ping;
>> -	}
>> +	return err;
>> +}
>>
>> -	if (!watchdog_active(wdd))
>> -		goto out_ping;
>> +static void watchdog_ping_work(struct work_struct *work)
>> +{
>> +	struct watchdog_device *wdd;
>>
>> -	if (wdd->ops->ping)
>> -		err = wdd->ops->ping(wdd);	/* ping the watchdog */
>> -	else
>> -		err = wdd->ops->start(wdd);	/* restart watchdog */
>> +	wdd = container_of(to_delayed_work(work), struct watchdog_device, work);
>>
>> -out_ping:
>> +	mutex_lock(&wdd->lock);
>> +	_watchdog_ping(wdd);
>> +	watchdog_update_worker(wdd, false);
>
> Here for the same reason you could pass true. So there is no caller that
> needs to pass false which allows to simplify the function. (i.e. drop
> the cancel parameter and simplify it assuming cancel is true)
>

There will be another call with 'false' added with a later patch, though
that could live with 'true'.

The function is executed by the worker, and since it is already executing
canceling it would not be necessary.

I don't know what happens if an attempt is made to cancel a worker from its
work function. I seem to recall that it causes a stall, but I may be wrong.
Any idea ?

>>   	mutex_unlock(&wdd->lock);
>> -	return err;
>>   }
>>
>>   /*
>> [...]
>> @@ -119,8 +134,9 @@ static inline void watchdog_set_nowayout(struct watchdog_device *wdd, bool noway
>>   /* Use the following function to check if a timeout value is invalid */
>>   static inline bool watchdog_timeout_invalid(struct watchdog_device *wdd, unsigned int t)
>>   {
>> -	return ((wdd->max_timeout != 0) &&
>> -		(t < wdd->min_timeout || t > wdd->max_timeout));
>
> Is this (old) code correct? watchdog_timeout_invalid returns false if
> wdd->max_timeout == 0 && t < wdd->min_timeout. I would have expected:
>
> 	return (wdd->max_timeout != 0 && t > wdd->max_timeout) ||
> 		t < wdd->min_timeout;
>
You are correct. However, that is a different problem, which I addressed in
'watchdog: Always evaluate new timeout against min_timeout'.

>> +	return t > UINT_MAX / 1000 ||
>> +		(!wdd->max_hw_timeout_ms && wdd->max_timeout &&
>> +		 (t < wdd->min_timeout || t > wdd->max_timeout));
>
> So should this better be:
>
> 	/* internal calculation is done in ms using unsigned variables */
> 	if (t > UINT_MAX / 1000)
> 		return 1;
>
> 	/*
> 	 * compat code for drivers not being aware of framework pings to
> 	 * bridge timeouts longer than supported by the hardware.
> 	 */
> 	if (!wdd->max_hw_timeout && wdd->max_timeout && t > wdd->max_timeout)
> 		return 1;
>
> 	if (t < wdd->min_timeout)
> 		return 1;
>

After all patches are applied, my code is

/* Use the following function to check if a timeout value is invalid */
static inline bool watchdog_timeout_invalid(struct watchdog_device *wdd, unsigned int t)
{
         return t > UINT_MAX / 1000 || t < wdd->min_timeout ||
                 (!wdd->max_hw_timeout_ms && wdd->max_timeout &&
                  t > wdd->max_timeout);
}

which is exactly the same (without the comments).

Thanks,
Guenter