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

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Date:	Thu, 29 May 2014 10:09:48 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	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>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Thu, 29 May 2014, Frederic Weisbecker wrote:

> > 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.
> 
> The lack of review may be partly due to a not very appealing changestat 
> on an old codebase that is already unpopular:
> 
>  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?

Yeah, printk() has grown over years to a stinking pile of you-know-what, 
no argument to that.

I also agree that performing a massive rewrite, which will make it use a 
lockless buffer, and therefore ultimately solve all its problems 
(scheduler deadlocks, NMI deadlocks, xtime_lock deadlocks) at once, is 
necessary in the long run.

On the other hand, I am completely sure that the diffstat for such rewrite 
is going to be much more scary :)

This is not adding fancy features to printk(), where we really should be 
saying no; horrible commits like 7ff9554bb5 is exactly something that 
should be pushed against *heavily*. But bugfixes for hard machine lockups 
are a completely different story to me (until we have a whole new printk() 
buffer handling implementation).

-- 
Jiri Kosina
SUSE Labs
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