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

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Hello Guenter,

On Tue, Sep 08, 2015 at 06:47:26AM -0700, Guenter Roeck wrote:
> 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'

The compile should be able to cope with that and only do the
multiplication once.

> (and to avoid a line > 80 columns in the first line).

unsigned timeout_ms = wdd->timeout * 1000; ?

> 
> >>[...]
> >>@@ -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 ?

No, I don't know if that works or not. But I would not expect any
problems.

> >>  	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'.

I usually consider it nice to have the fixes first in the series. I
didn't look into the later patches yet. This should be fixed for 4.3.

> >>+	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).

The comments make it a tad nicer though :-)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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