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

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Date:	Fri, 30 May 2014 12:10:06 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Jan Kara <jack@...e.cz>
cc:	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kay Sievers <kay@...y.org>, Michal Hocko <mhocko@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Fri, 30 May 2014, Jan Kara wrote:

> >  Documentation/kernel-parameters.txt |   19 +-
> >  kernel/printk/printk.c              | 1218 +++++++++++++++++++++++++----------
> >  2 files changed, 878 insertions(+), 359 deletions(-)
> > 
> > 
> > Your patches look clean and pretty nice actually. They must be seriously
> > considered if we want to keep the current locked ring buffer design and
> > extend it to multiple per context buffers. But I wonder if it's worth to
> > continue that way with the printk ancient design.
> > 
> > If it takes more than 1000 line changes (including 500 added) to make it
> > finally work correctly with NMIs by working around its fundamental flaws,
> > shouldn't we rather redesign it to use a lockless ring buffer like ftrace
> > or perf ones?
>   I agree that lockless ringbuffer would be a more elegant solution but a
> much more intrusive one and complex as well. Petr's patch set basically
> leaves ordinary printk path intact to avoid concerns about regressions
> there.

Fully agreed, vast majority of the changes done by the patchset are on the 
unlikely in-NMI path, leaving normal printk operation as-is.

-- 
Jiri Kosina
SUSE Labs
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