linux-kernel - Re: [GIT PULL] tracing/NMI/printk: Use seq_buf for safe printing from NMI context

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Message-ID: <20141208100851.3906cc0a@gandalf.local.home>
Date:	Mon, 8 Dec 2014 10:08:51 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Kosina <jkosina@...e.cz>, Petr Mladek <pmladek@...e.cz>
Subject: Re: [GIT PULL] tracing/NMI/printk: Use seq_buf for safe printing
 from NMI context

On Mon, 8 Dec 2014 10:05:28 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:


> This code solves the issue of performing stack dumps from NMI context.
> The issue is that printk() is not safe from NMI context as if the NMI
> were to trigger when a printk() was being performed, the NMI could
> deadlock from the printk() internal locks. This has been seen in practice.
> 

One added bonus is that this code also makes the NMI dump stack work on
PREEMPT_RT kernels. As printk() includes sleeping locks on PREEMPT_RT,
printk() only writes to console if the console does not use any
rt_mutex converted spin locks. Which a lot do.

-- Steve
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