linux-kernel - Re: [PATCH] watchdog: Respect handle_boot_enabled when setting last last_hw

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Message-ID: <def0c2e9-e035-7ffc-3216-27f461555ae5@siemens.com>
Date:   Fri, 30 Jul 2021 23:37:27 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Guenter Roeck <linux@...ck-us.net>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        linux-watchdog@...r.kernel.org
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tero Kristo <t-kristo@...com>
Subject: Re: [PATCH] watchdog: Respect handle_boot_enabled when setting last
 last_hw_keepalive

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

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