linux-kernel - Re: [PATCH] lockdep: Avoid triggering hardlockup from debug_show_all

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Message-ID: <20180123211154.GI1771050@devbig577.frc2.facebook.com>
Date:   Tue, 23 Jan 2018 13:11:54 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Rik van Riel <riel@...com>, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-team@...com,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH] lockdep: Avoid triggering hardlockup from
 debug_show_all_locks()

Hello, Steven.

On Tue, Jan 23, 2018 at 04:00:54PM -0500, Steven Rostedt wrote:
> On Tue, 23 Jan 2018 12:57:06 -0800
> Tejun Heo <tj@...nel.org> wrote:
> 
> > Yeah, it's ridiculous how often printk ends up escalating otherwise
> > recoverable situations into system crashes.  I don't know what the
> > right answer is.  For spurious NMI hardlockups, maybe auditing debug
> > paths and adding touch_nmi_watchdog() would be enough but that also is
> > a pretty leaky approach.
> 
> What about if every printk were to touch NMI watchdog?
> 
> NMI watchdog is really there for when the system locks up. If the
> system is locked up doing printk, at least we see what is happening,
> and not a total freeze.

Yeah, that would definitely be a solution.  The downside is that when
the system completely locks up from printk storm while holding
critical locks (say, tasklist_lock), the watchdog won't be able to
reset the system.  I guess the judgement would depend on what one
expects of the NMI watchdog, but I personally would be happier with
printk touching NMI automatically.

Thanks.

-- 
tejun