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

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Message-ID: <alpine.LNX.2.00.1405290000350.17241@pobox.suse.cz>
Date:	Thu, 29 May 2014 00:02:30 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Petr Mladek <pmladek@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kay Sievers <kay@...y.org>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Fri, 9 May 2014, Petr Mladek wrote:

> printk() cannot be used safely in NMI context because it uses internal locks
> and thus could cause a deadlock. Unfortunately there are circumstances when
> calling printk from NMI is very useful. For example, all WARN.*(in_nmi())
> would be much more helpful if they didn't lockup the machine.
> 
> Another example would be arch_trigger_all_cpu_backtrace for x86 which uses NMI
> to dump traces on all CPU (either triggered by sysrq+l or from RCU stall
> detector).

I am rather surprised that this patchset hasn't received a single review 
comment for 3 weeks.

Let me point out that the issues Petr is talking about in the cover letter 
are real -- we've actually seen the lockups triggered by RCU stall 
detector trying to dump stacks on all CPUs, and hard-locking machine up 
while doing so.

So this really needs to be solved.

-- 
Jiri Kosina
SUSE Labs
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