Re: [PATCH] powerpc/watchdog: disable watchdog when watchdog_thresh=0

[Michael Ellerman <mpe@tenstorrent.com>]

On 4/11/26 2:40 pm, Luiz Capitulino (Red Hat) wrote:
> On powerpc, trying to disable the hardlockup detector with:
> 
>   # echo 0 > /proc/sys/kernel/watchdog_thresh
> 
> causes an immediate and endless hardlockup detection, eventually leading
> to an unusable system (splat example below).

...
> This condition is already checked in __lockup_detector_reconfigure() for
> the soft lockup detector and the arch-independent hardlockup detector,
> what's missing is powerpc performing the same check in start_watchdog().

Arguably setting watchdog_thresh to 0 should mean watchdog_enabled becomes false.
Then there wouldn't need to be extra checks for thresh == 0 anywhere.

But if the existing code already checks for thresh == 0 then this is fine.

> Fixes: 2104180a5369 ("powerpc/64s: implement arch-specific hardlockup watchdog")
> Cc: stable@vger.kernel.org
> Signed-off-by: Luiz Capitulino (Red Hat) <luiz.capitulino@linux.dev>
> ---
> Notes:
> - This issue was not discovered by AI. We have a new test-case in our CI that
>    sets watchdog_thresh=0, when running on powerpc we get the described issue
> - This issue was initially reproduced on Power10, but I developed the fix
>    on Power9 (which is what I have access now)
> 
>   arch/powerpc/kernel/watchdog.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
> index 582ee3fd71c1..44f178e07531 100644
> --- a/arch/powerpc/kernel/watchdog.c
> +++ b/arch/powerpc/kernel/watchdog.c
> @@ -487,6 +487,9 @@ static void start_watchdog(void *arg)
>   	if (!(watchdog_enabled & WATCHDOG_HARDLOCKUP_ENABLED))
>   		return;
>   
> +	if (!watchdog_thresh)
> +		return;
> +
>   	if (!cpumask_test_cpu(cpu, &watchdog_cpumask))
>   		return;


Fix looks good to me.

Reviewed-by: Michael Ellerman <mpe@kernel.org>

cheers