Re: [PATCH] watchdog: Respect handle_boot_enabled when setting last last_hw_keepalive

[Guenter Roeck]

On 7/30/21 2:37 PM, Jan Kiszka wrote:
> On 30.07.21 22:49, Guenter Roeck wrote:
> > On 7/30/21 12:39 PM, Jan Kiszka wrote:
> > > From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > >
> > > We must not pet a running watchdog when handle_boot_enabled is off
> > > because this requests to only start doing that via userspace, not during
> > > probing.
> >
> > The scope of the changed function is quite limited. See the
> > definition of watchdog_set_last_hw_keepalive(). On top of that,
> > __watchdog_ping() does a bit more than just ping the watchdog,
> > and it only pings the watchdog in limited circumstances. On top of that,
> > the scope of handle_boot_enabled is different: If enabled, it tells
> > the watchdog core to keep pinging a watchdog until userspace opens
> > the device. This is about continuous pings, not about an initial one.
> > Given that, I'd rather have the watchdog subsystem issue an additional
> > ping than risking a regression.
> >
> > The only driver calling watchdog_set_last_hw_keepalive() is rti_wdt.c.
> > Does this patch solve a specific problem observed with that watchdog ?
>
> Yes, it unbreaks support for handle_boot_enabled=no by not starting the
> automatic pinging of the kernel until userspace opens the device.
> Without this fix, the core will prematurely start kernel-side pinging,
> and hanging userspace will never be detected.
Good point. You are correct.

I think it should also check for watchdog_hw_running(wdd), though.
The function should not really be called if the watchdog isn't
running, but it should still not ping the watchdog in that case.

Something like

    if (watchdog_hw_running(wdd) && handle_boot_enabled)
	return __watchdog_ping(wdd);

    return 0;

Also, I think it would make sense to add your additional comment
to the patch description. The problem isn't only that the watchdog
is pinged once, the problem is that it starts _automatic_
pinging which it really should not do.

Thanks,
Guenter