linux-kernel - Re: [PATCH 0/4] Convert timekeeping core to use printk

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Date:	Fri, 2 May 2014 16:05:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Bohac <jbohac@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 0/4] Convert timekeeping core to use printk_deferred
 (v2)

On Fri,  2 May 2014 15:09:14 -0700 John Stultz <john.stultz@...aro.org> wrote:

> Recently, Jiri pointed out a potential deadlock when calling printk
> while holding the timekeeping seqlock.
> 
> Annoyingly, the seqlock lockdep enablement doesn't catch this, as
> printk disables lockdep.
> 
> When looking for possible solutions, one idea was to use a local buffer
> and defer the printk to later. Ends up there is already similar
> functionality in printk_sched() to avoid similar style deadlocks w/
> the scheduler.
> 
> Thus this patchset (based on next/akpm) renames printk_sched to
> printk_deferred and then moves the affected timekeeping printks to make
> use of it.
> 
> There were some points in the discussion between Jan and Peter that
> made it seem that there may still be problems lurking in the console
> layer, and I'm not sure I fully understand their point, so this solution
> may be incomplete.
> 
> Additionally, the same issue likely affects any WARN_ONs as well, but
> I wanted to get some thoughts on this approach before trying to remove
> or convert affected WARN_ONS.
> 
> Your thoughts and feedback are greatly appreciated!

All look pretty simple and sane to me.  printk is a crazy hotspot
lately but this patchset looks like it won't get singed.

Would "printk_deferred_once" be more logical than
"printk_once_deferred"?  Think so.  It's (((printk(deferred(once))),
not (((printk(once(deferred))).


Why do I see a pr_emerg_once_deferred() in my future?
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