Date: Thu, 15 Dec 2016 18:41:15 +0000
From: Aaron Tomlin <atomlin@redhat.com>
To: Don Zickus <dzickus@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ulrich Obergfell <uobergfe@redhat.com>
Subject: Re: [PATCH] kernel/watchdog: Prevent false hardlockup on overloaded
 system
Message-ID: <20161215184114.ut2ulhhflap5bfur@atomlin.usersys.redhat.com>
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On Tue 2016-12-06 11:17 -0500, Don Zickus wrote:
> On an overloaded system, it is possible that a change in the watchdog threshold
> can be delayed long enough to trigger a false positive.
> 
> This can easily be achieved by having a cpu spinning indefinitely on a task,
> while another cpu updates watchdog threshold.
> 
> What happens is while trying to park the watchdog threads, the hrtimers on the
> other cpus trigger and reprogram themselves with the new slower watchdog
> threshold.  Meanwhile, the nmi watchdog is still programmed with the old faster
> threshold.
> 
> Because the one cpu is blocked, it prevents the thread parking on the other
> cpus from completing, which is needed to shutdown the nmi watchdog and
> reprogram it correctly.  As a result, a false positive from the nmi watchdog is
> reported.
> 
> Fix this by setting a park_in_progress flag to block all lockups
> until the parking is complete.
> 
> Fix provided by Ulrich Obergfell.
> 
> Cc: Ulrich Obergfell <uobergfe@redhat.com>
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
>  include/linux/nmi.h   | 1 +
>  kernel/watchdog.c     | 9 +++++++++
>  kernel/watchdog_hld.c | 3 +++
>  3 files changed, 13 insertions(+)

Looks fine to me.

Reviewed-by: Aaron Tomlin <atomlin@redhat.com>

-- 
Aaron Tomlin