linux-kernel - Re: [PATCH 0/8] watchdog: Add support for keepalives triggered by infrastructure

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Date:	Wed, 5 Aug 2015 12:13:49 -0500
From:	David Teigland <teigland@...hat.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-watchdog@...r.kernel.org, Wim Van Sebroeck <wim@...ana.be>,
	linux-kernel@...r.kernel.org,
	Timo Kokkonen <timo.kokkonen@...code.fi>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, linux-doc@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 0/8] watchdog: Add support for keepalives triggered by
 infrastructure

On Mon, Aug 03, 2015 at 07:13:26PM -0700, Guenter Roeck wrote:
> - Some watchdogs have a very short maximum timeout, in the range of just a few
>   seconds. Such low timeouts are difficult if not impossible to support from
>   user space. Drivers supporting such watchdog hardware need to implement
>   a timer function to augment heartbeats from user space.

> - A new status flag, WDOG_RUNNING, informs the watchdog subsystem that a
>   watchdog is running, and that the watchdog subsystem needs to generate
>   heartbeat requests while the associated watchdog device is closed.

> Patch #2 adds timer functionality to the watchdog core. It solves the problem
> of short maximum hardware timeouts by augmenting heartbeats triggered from
> user space with internally triggered heartbeats.
> 
> Patch #3 adds functionality to generate heartbeats while the watchdog device is
> closed. It handles situation where where the watchdog is running after
> the driver has been instantiated, but the device is not yet opened,
> and post-close situations necessary if a watchdog can not be stopped.

These sound concerning because it seems that heartbeats could be generated
outside of the direct control of userspace.  I have a program that depends
on having direct control over whether heartbeats are generated (or more
specifically, *not* generated.)  If these new features introduce a new way
for heartbeats to be generated, is there a way I can detect or disable
that behavior from userspace?  Unwanted heartbeats could break my program
and may lead to data corruption.

A related issue from some years ago is the unfortunate fact that closing
the watchdog device also generates a heartbeat.  I'd like to disable that
also, and submitted a patch for it here:
http://www.spinics.net/lists/linux-watchdog/msg01477.html

(Without the patch, I have to work around it by closing the device
prematurely as a way to generate the potentially final heartbeat, and then
reopen it again if I want to continue the heartbeats.)

Dave
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