linux-kernel - Re: [RFC PATCH 00/11] printk: safe printing in NMI context

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Message-ID: <20140618144457.GF4669@linux.vnet.ibm.com>
Date:	Wed, 18 Jun 2014 07:44:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Petr Mladek <pmladek@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	Kay Sievers <kay@...y.org>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Wed, Jun 18, 2014 at 04:41:09PM +0200, Jiri Kosina wrote:
> On Wed, 18 Jun 2014, Paul E. McKenney wrote:
> 
> > > - both RCU stall detector and 'echo l > sysrq-trigger' can (and we've 
> > >   seen it happening for real) cause a complete, undebuggable, silent hang 
> > >   of machine (deadlock in NMI context)
> > 
> > I could easily add an option to RCU to allow people to tell it not to
> > use NMIs to dump the stack.  Would that help?
> 
> Well, that would make unfortunately the information provided by RCU stall 
> detector rather useless ... workqueue-based stack dumping is very unlikely 
> to point its finger to the real offender, as it'd be coming way too late.

I would not use workqueues, but rather have the CPU detecting the
stall grovel through the other CPUs' stacks, which is what I do now for
architectures that don't support NMI-based stack dumps.  Would that be
a reasonable approach?

							Thanx, Paul

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