[PATCH] watchdog: Respect handle_boot_enabled when setting last last_hw_keepalive

[PATCH] watchdog: Respect handle_boot_enabled when setting last last_hw_keepalive

[Jan Kiszka]

From: Jan Kiszka <jan.kiszka@siemens.com>

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.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 drivers/watchdog/watchdog_dev.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 3bab32485273..3c93d00bb284 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -1172,7 +1172,10 @@ int watchdog_set_last_hw_keepalive(struct watchdog_device *wdd,
 
 	wd_data->last_hw_keepalive = ktime_sub(now, ms_to_ktime(last_ping_ms));
 
-	return __watchdog_ping(wdd);
+	if (handle_boot_enabled)
+		return __watchdog_ping(wdd);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(watchdog_set_last_hw_keepalive);
 
-- 
2.31.1

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

[Guenter Roeck]

On 7/30/21 12:39 PM, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka@siemens.com>
> 
> 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 ?

Guenter

> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>   drivers/watchdog/watchdog_dev.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index 3bab32485273..3c93d00bb284 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -1172,7 +1172,10 @@ int watchdog_set_last_hw_keepalive(struct watchdog_device *wdd,
>   
>   	wd_data->last_hw_keepalive = ktime_sub(now, ms_to_ktime(last_ping_ms));
>   
> -	return __watchdog_ping(wdd);
> +	if (handle_boot_enabled)
> +		return __watchdog_ping(wdd);
> +
> +	return 0;
>   }
>   EXPORT_SYMBOL_GPL(watchdog_set_last_hw_keepalive);
>   
> 

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

[Jan Kiszka]

On 30.07.21 22:49, Guenter Roeck wrote:
> On 7/30/21 12:39 PM, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> 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.

Jan

-- 
Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux

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@siemens.com>
>>>
>>> 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